First-class HTTP header support

[Xorlev]

First-class HTTP header support. HTTP request headers are read into and attached to the gRPC Context. Likewise, response headers can be controlled from within your RPC handler.

This works by capturing the HTTP request headers from Jersey. These headers are given to a gRPC ClientInterceptor which translates these headers into a protocol buffer message (imaginatively named Headers). This message is then attached as a Metadata key and sent along with the underlying RPC.

A ServerInterceptor extracts this protocol buffer message and writes the headers into the current gRPC Context (smart thread-local state).

The RPC handler can read the headers (separate from the headers being splatted into the Metadata, as is the unfortunate default of the "proxy" mode) and do what it wants with them. ServerInterceptors that run after the HttpHeaderServerInterceptor can also work with these headers (e.g. extract OAuth tokens, JWT, etc.) to transform those headers into whatever Metadata or Context attachment you like.

All headers are available inside the HttpHeaderContext, a static utility for working with Context-local state. HttpHeaderContext.requestHeaders() gives access to the request headers, and there's various setter/clear methods for response headers, e.x. HttpHeaderContext.setResponseHeader("X-Hello", "World"). Likewise, ServerInterceptors running before the HttpHeaderServerInterceptor can also set headers. And, just like before, these headers are bundled up into the Headers proto and sent to the client (usually in-process) to be set on the outgoing HTTP response.

TODO:

• Stabilize integration tests. GrizzlyWebTestContainerFactory seems to have some behavioral differences that aren't deterministic -- a test will fail. Run that one test, it works. The streaming tests don't work at all, despite working via manual testing with the ProxyIntegrationApp.
• Update ServerStub tests to not check HTTP features.
• Return headers on a best-effort basis from RPC errors.

[Xorlev]

FYI @smartwjw