Add JAXB Converter in Document

Functionally does nothing, just somewhat cleaner to be able to do `baseUrl(URL)` than `baseUrl(URL#toString());` if handling a `URL` rather than `String`.
Used `HttpUrl.get(String)` as `HttpUrl.get(URL)` returns null instead of throwing an exception.

Support okhttp3.Headers for HeadersMap parameters.

Next version is 2.6.0

Also fix some Javadoc errors and ensure Javadoc (and source) jar generation is tested on CI.

Update to Gson 2.8.5.

Fix support for protobuf 3.5+

Remove use of Mockito

It wasn't used for anything interesting and I prefer not having it around to tempt contributors to add new usages of it.

Add some missing @Nullable annotations to Utils

Back suspend methods with the CallAdapter for Call.

This allows the regular mechanism of composition of behavior without explicitly needing to model suspending functions in the API.

Add annotation for skipping the callback executor

Do not leak body in intermediate buffers to converters

Multiple calls to source() would create a new BufferedSource which, when a single byte was read, would cache data in its buffer which would be lost on a subsequent call to source(). Calls to source() will now always return the same BufferedSource ensuring data cannot be lost.

Test canceling a suspend invocation propagates to Call

Add parameter annotation @Tag for setting request tags

Add first-party Kotlin coroutine suspend support

Add more attributes to Retrofit's R8/ProGuard rules

Usually these are supplied by things like the defaults from the Android SDK, but if you are not using those then Retrofit will fail. Seems like we should be including a complete set rather than assuming any defaults.

Mark HttpException.response() as nullable

Fatal errors should crash the dispatcher thread

Ensure JAX-B converter closes its body on error

Keep Retrofit interfaces in R8 full mode