provide a rhai interface to the router service (#3234)

Adds `Rhai` support for the `router_service`.

It is now possible to interact with requests and responses at the
`router_service` level from `Rhai`. The functionality is very similar to
that provided for interacting with existing services, for example
`supergraph_service`. For instance, you may map requests and responses
as follows:

```
fn router_service(service) {
    const request_callback = Fn("process_request");
    service.map_request(request_callback);
    const response_callback = Fn("process_response");
    service.map_response(response_callback);
}

```
The main difference from existing services is that the router_service is
dealing with HTTP Bodies, not well formatted GraphQL objects. This means
that the `Request.body` or `Response.body` is not a well structured
object that you may interact with, but is simply a String.

This makes it more complex to deal with Request and Response bodies with
the tradeoff being that a script author has more power and can perform
tasks which are just not possible within the confines of a well-formed
GraphQL object.

This simple example, simply logs the bodies:

```
// Generate a log for each request at this stage
fn process_request(request) {
    print(`body: ${request.body}`);
}

// Generate a log for each response at this stage
fn process_response(response) {
    print(`body: ${response.body}`);
}
```

Fixes #2278 

---------

Co-authored-by: Geoffroy Couprie <[email protected]>
Co-authored-by: Bryn Cooke <[email protected]>

.changesets/feat_garypen_2278_rhai_router_service.md

@@ -0,0 +1,43 @@
+### Provide a rhai interface to the router service ([Issue #2278](https://github.com/apollographql/router/issues/2278))
+
+Adds `Rhai` support for the `router_service`.
+
+It is now possible to interact with requests and responses at the `router_service` level from `Rhai`. The functionality is very similar to that provided for interacting with existing services, for example `supergraph_service`. For instance, you may map requests and responses as follows:
+
+```rust
+fn router_service(service) {
+    const request_callback = Fn("process_request");
+    service.map_request(request_callback);
+    const response_callback = Fn("process_response");
+    service.map_response(response_callback);
+}
+
+```
+The main difference from existing services is that the router_service is dealing with HTTP Bodies, not well formatted GraphQL objects. This means that the `Request.body` or `Response.body` is not a well structured object that you may interact with, but is simply a String.
+
+This makes it more complex to deal with Request and Response bodies with the tradeoff being that a script author has more power and can perform tasks which are just not possible within the confines of a well-formed GraphQL object.
+
+This simple example, simply logs the bodies:
+
+```rust
+// Generate a log for each request at this stage
+fn process_request(request) {
+    print(`body: ${request.body}`);
+}
+
+// Generate a log for each response at this stage
+fn process_response(response) {
+    print(`body: ${response.body}`);
+}
+```
+
+This PR also introduces two new Rhai functions:
+
+```rust
+json_encode(Object)
+json_decode(String) -> Object
+
+```
+Which will respectively encode a `Rhai` Object or decode a JSON string into a `Rhai` Object. These functions may be helpful when dealing with String bodies which represent encoded JSON objects.
+
+By [@garypen](https://github.com/garypen) in https://github.com/apollographql/router/pull/3234