Add Relaxed Data Binding Proposal to the Docs

docs/proposals/introducing_relaxed_data_binding.md

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+# Proposal: Introduce Relaxed Data Binding to Ballerina HTTP Module
+
+_Owners_: @shafreenAnfar @daneshk @NipunaRanasinghe @lnash94 @TharmiganK @SachinAkash01  
+_Reviewers_: @lnash94 @TharmiganK @shafreenAnfar    
+_Created_: 2024/11/13  
+_Updated_: 2024/11/18  
+_Issue_: [#7366](https://github.com/ballerina-platform/ballerina-library/issues/7366)
+
+## Summary
+
+Using the projection support from conversion APIs from the data module, the HTTP module will add a fix by providing a configurable to handle undefined nullable property scenarios where the user can choose strict data binding or relaxed data binding.
+
+## Goals
+- Enhance the Ballerina HTTP module by adding support to handle undefined nullable properties where the users can choose strict data binding or relaxed data binding.
+
+## Motivation
+According to OpenAPI specification if a given property is required or/and nullable we need to specifically mention it as explained below.
+
+If a given property is `required`,
+```yaml
+type: object
+properties:
+  id:
+    type: integer
+  username:
+    type: string
+required:
+  - id
+  - username
+```
+
+If a given property is `nullable`,
+```yaml
+type: integer
+nullable: true
+```
+
+Therefore if a given property is not specifically mentioned as required, then that property is optional by default. Similarly if a given property is not specifically mentioned as `nullable: true`, then that property is not accepting null values by default.
+
+There are several scenarios where the received response has null values even though that property is not configured as `nullable: true`. In such cases http target type binding fails, returning a type conversion error.
+
+As a workaround, there is a `--nullable` flag to make all the fields nullable when it is not defined in the OpenAPI specification. However this approach also makes all the fields nullable which is not an ideal solution.
+
+A more user-friendly solution is to implement relaxed data binding, which would allow the tool to accept null values for non-nullable fields without raising runtime errors. This approach would handle unexpected null values gracefully, reducing type conversion issues and enhancing the developer experience by aligning more closely with API responses.
+
+## Description
+Following is the breakdown of the cases we need to reconsider within the context of payload data binding:
+| Scenario                | Ballerina Record Field Representation | Default Behavior                                          | Relaxed Behavior                                              |
+|-------------------------|---------------------------------------|-----------------------------------------------------------|---------------------------------------------------------------|
+| Required & non-nullable | `T foo;`                              | Both null values and absent fields cause runtime failures | Same                                                          |
+| Required & nullable     | `T? foo;`                             | Absent fields will cause runtime failures                 | Need relaxed data binding to map absent fields as nil values  |
+| Optional & non-nullable | `T foo?;`                             | Null values will cause runtime failures                   | Need relaxed data binding to map null values as absent fields |
+| Optional & nullable     | `T? foo?;`                            | Both nil values and absent fields are allowed             | Same                                                          |              |
+
+In the `data.jsondata` package, the `Options` configuration already provides support for handling various field types in JSON data binding, accommodating scenarios with both absent fields and null values. The `Options` record allows us to adjust how nullability and optionality are treated, making it a versatile solution for managing the differences between required and optional fields, as well as nullable and non-nullable fields.
+
+The `Options` record in the `data.jsondata` module is defined as follows:,
+
+```ballerina
+public type Options record {
+    record {
+        # If `true`, nil values will be considered as optional fields in the projection.
+        boolean nilAsOptionalField = false;
+        # If `true`, absent fields will be considered as nilable types in the projection.
+        boolean absentAsNilableType = false;
+    }|false allowDataProjection = {};
+    boolean enableConstraintValidation = true;
+};
+```
+
+This configuration offers flexibility through two primary settings,
+- `nilAsOptionalField`: when set to `true`, null values are treated as optional fields, allowing relaxed data binding for Optional and Non-Nullable cases. This way, null values in the input can be mapped into an absent field, avoiding runtime failures.
+- `absentAsOptionalField`: when enabled, absent fields are interpreted as nullable, providing support for Required & Nullable cases by mapping absent fields into null values  instead of causing runtime errors.
+
+### Configure Relaxed Data Binding in Client Level
+The approach involves adding a new field, `laxDataBinding`, to the `CommonClientConfiguration`. When set to true, it enables relaxed data binding for null and absent fields in the JSON payload by internally mapping to both `nilAsOptionalField` and `absentAsNilableType` in the `data.jsondata` module. This option will be enabled by default for clients generated using the Ballerina OpenAPI tool.
+
+```ballerina
+public type CommonClientConfiguration record {|
+    boolean laxDataBinding = false;
+    // Other fields
+|};
+```
+
+### Configure Relaxed Data Binding in Service Level
+We can configure the relaxed data binding in the service level by improving the `HttpServiceConfig` configuration by introducing a new boolean field called `laxDataBinding`, similar to the approach used in the client configuration.
+
+```ballerina
+public type HttpServiceConfig record {|
+    boolean laxDataBinding = false;
+    // Other fields
+|};
+```