Add extensions for ResponseMessage like Kotlin's Result type

[kohenkatz]

The ResponseMessage type is very similar to Kotlin's built-in Result type. The only major difference is that the ResponseMessage type has additional fields for response code, headers, and trailers.

The Kotlin Result type has a number of convenient extension functions which can simplify many common coding situations. For example, getOrThrow is useful for situations where a code block is already surrounded by a try/catch, and getOrNull/getOrDefault are useful for situations in which a failure is not critical. Other places in this library already contain fold methods similar to the Result type.

This PR adds extension functions to ResponseMessage that match the ones present on Result.

I'm not sure what type of tests are appropriate for these functions, if any, considering that I don't see any tests for the Kotlin Result implementation of these functions.

[CLAassistant]

All committers have signed the CLA.

[buildbreaker]

This is a reasonable add. Thanks for your help @kohenkatz !