Make library build process match other authzed libs

[tstirrat15]

Description

Most of our other libs, including those that have static compilation (like csharp), use buf generate to go directly from remote proto to generated code. This lib is currently using buf export and the gradle build process to run the compilation steps locally.

It'd be nice if this matched other libraries.

It'd also make it easier to inspect the library code and understand what's going on, since the generated source code would be visible and names of various classes would be inspectable. That was something painful to me as a consumer.

[tstirrat15]

I've got a start on this in the 100-match-other-build-processes branch; I think the hardest part will be reliably providing a reference to the protoc-gen-grpc-java binary. I'm going to pause on this for now.