Add template-level parameters and file_name per template config

[lquerel]

This PR adds two new optional fields in the templates section of a Weaver configuration file:

• params: Defines a map of key-value pairs which will be merged with the ones defined at the top level of the weaver.yaml file. CLI parameters can also be used to override these parameters.
• file_name: Defines the relative file path of the output generated from the template. This field is a Jinja expression which will be evaluated with the same context used in the template itself. The parameters mentioned above are also accessible.

An example of usage:

params:
  exclude_deprecated: true
  
templates:
  - pattern: attribute.py
    params:
      exclude_stability: ["stable"]
    filter: "semconv_grouped_attributes($params)"
    application_mode: each
    file_name: "incubating/{{ctx.root_namespace | snake_case}}.py"
  - pattern: attribute.py
    params:
      exclude_stability: ["experimental"]
    filter: "semconv_grouped_attributes($params)"
    application_mode: each
    file_name: "{{ctx.root_namespace | snake_case}}.py"

Note: params are now merged in an additive way. For example, if a parameter is defined in the params section of a template, then the final params section for this template will include both the params inherited from the file-level configuration and the params defined in the template configuration. If the override is a parameter set to null, then this parameter is removed from the final params section. This modification should not have a significant impact on the existing codegen, as I don't believe any codegen was relying on this specific behavior.

Closes open-telemetry/weaver#288.

[jsuereth]

Overall looks good BUT I find the weaver config confusing.

I think we should deprecated pattern for template.

Specifically:

templates:
  - pattern: test.md
    filter: .
    application_mode: single
    params:
      template_name: test1
      test_1: true
    file_name: "{{params.template_name | snake_case}}.json"

Looks confusing, since test.md is the template you're using, not a pattern at all.

Instead, let's do:

templates:
  - template: test.md
    filter: .
    application_mode: single
    params:
      template_name: test1
      test_1: true
    file_name: "{{params.template_name | snake_case}}.json"

I think we can do this in a non-breaking way by deprecating the "pattern" attribute and adding "template" where we pull from pattern if it exists, otherwise use template.

[lquerel]

I think we can do this in a non-breaking way by deprecating the "pattern" attribute and adding "template" where we pull from pattern if it exists, otherwise use template.

Can be done with a serde alias. I will also update the docs.

[lquerel]

@jsuereth I renamed templates.pattern to templates.template and added pattern as a serde alias of template for backward-compatibility. I also updated documentations and examples.