feat(build): Add CodeGenBuilder (#1154)

This commit adds a new `CodeGenBuilder` that replaces the
client/server generate fn with a builder stlye that allows
adding config items in an non-breaking way. This also deprecates
both of the client/server generate fn in favor of the builder ones.

tonic-build/src/lib.rs

@@ -94,6 +94,9 @@ pub mod client;
 /// Service code generation for Server
 pub mod server;
 
+mod code_gen;
+pub use code_gen::CodeGen8uilder;
+
 /// Service generation trait.
 ///
 /// This trait can be implemented and consumed