Lock down JAXB. Don't load remote entities.

XML is subject to XXE attacks.
https://www.owasp.org/index.php/XML_External_Entity_(XXE)_Prevention_Cheat_Sheet#JAXB_Unmarshaller

Makes sure observable assembly hooks are called

This allows things like error and trace tracking to work naturally.

Fixes #2716

Small tweaks with exceptions.

Document that JAXB doesn't work on Android

Add a test for not overwriting headers.

To break the test, use Headers.Builder.set instead of add in ServiceMethod.

Fix local converter name.

Don't ever pass fatal errors to the callback.

Replace 'compile' with 'implementation

* Add missing nullability annotations.

* Inline single-use utility methods.

* Remove redundant throws.

* Inline method only called once.

Use HTTPS links in documentation where possible.

Closes: https://github.com/square/retrofit/issues/2425

Update to OkHttp 3.10.0.

Assert full JSON document consumption with Moshi.

Assert full JSON document consumption with Gson.

Update Scala dependency