Support one-time configuration of expression parameters

Problem

Let's say I have a request template that looks like this:

@RequestLine("GET /{env}/{resource}")
void getResource(@Param("env") String env, @Param("resource") String resource);

In the application that defines this template, env is configured at startup and will only ever have 1 value (e.g. "production" or "qa") during the application's lifespan. It would make sense to not have to pass this same value into each call of getResource().

Default methods are of no help here because env still needs to be dynamically set.

Proposal

Support on-time configuration of expression parameters via configuration methods on the client interface:

@Configuration
void setEnv(@Param("env") String env);

@RequestLine("GET /{env}/{resource}")
void getResource(@Param("resource") String resource);

Implementing this is pretty easy (~150 lines).

N.B. this proposal is somewhat similar to https://github.com/OpenFeign/feign/issues/1359.

No files, tests, or entry points are named. Start by tracing how Feign processes @RequestLine, @Param, and interface configuration methods, then determine where a configured parameter can be retained and applied to later calls; done means one-time configuration removes that value from each request method's arguments.