feat: Add REST Interceptors to support reading metadata

[parthea]

Fixes #2263 🦕

[vchudnov-g]

Consider my wording tweaks before merging.

[vchudnov-g]

I suggest

        DEPRECATED. Please use the `post_{{ method.name|snake_case }}_with_metadata`
        interceptor instead. 

        Override in a subclass to read or manipulate the response
        after it is returned by the {{ service.name }} server but before
        it is returned to user code. This `post_{{ method.name|snake_case }}` interceptor runs
        before the `post_{{ method.name|snake_case }}_with_metadata` interceptor.

(reworded slightly and fixed spelling of "interceptor")

[parthea]

Fixed in 83cfa50. Good catch on the spelling error! I filed #2301 to see if we can add a presubmit to help with future development.

[vchudnov-g]

Suggested change

	Override in a subclass to read or manipulate the response or metadata after it
	is returned by the {{ service.name }} server but before it is returned to user code.
	This `post_{{ method.name|snake_case }}_with_metadata` interceptor runs after the
	`post_{{ method.name|snake_case }}` iterceptor. If the response is modified in
	`post_{{ method.name|snake_case }}`, the modified response will be used in
	`post_{{ method.name|snake_case }}_with_metadata`. This `post_{{ method.name|snake_case }}_with_metadata`
	interceptor is recommended for new development instead of `post_{{ method.name|snake_case }}`.
	Override in a subclass to read or manipulate the response or metadata after it
	is returned by the {{ service.name }} server but before it is returned to user code.
	We recommend only using this `post_{{ method.name|snake_case }}_with_metadata`
	interceptor in new development instead of the `post_{{ method.name|snake_case }}` interceptor.
	When both interceptors are used, this `post_{{ method.name|snake_case }}_with_metadata` interceptor runs after the
	`post_{{ method.name|snake_case }}` interceptor. The (possibly modified) response returned by
	`post_{{ method.name|snake_case }}` will be passed to
	`post_{{ method.name|snake_case }}_with_metadata`.

[parthea]

Done in 11b803f