Support Bindable annotation for request body arguments in declarative client

[ondrej-hrstka]

Hi,

when using declarative http client, the method arguments names are the ones, which are used in the request body. But sometimes I might need to make request and use different name that is the argument name. Would it make sense accept Bindable annotation in processing the name? Looking in the source code, it should be pretty simple change in HttpClientIntroductionAdvice.

@Client("some-service")
interface ClientOperations {
        @Post("doPost")
    fun doPost(@Body("first_argument") firstArgument: String, @Body("second_argument") secondArgument: String): String
}

In the example above, the client would make request with body

{
  "first_argument": "....",
  "second_argument": "...."
}

Does this makes sense? I can prepare PR with the change, but wanted to discuss the idea first.