Implement compression interop tests

[ericgribkoff]

There are four compression interop tests defined in interop-test-descriptions.md but not implemented for Java:

• client_compressed_unary
• client_compressed_streaming
• server_compressed_unary
• server_compressed_streaming

These should be implemented and enabled in run_interop_tests.py.

[ejona86]

server_compressed_unary and server_compressed_streaming cannot be implemented in Java, because the compressed message flag is not available and we have no want to expose such a feature. Last I heard, the cross-language expectation is that only C++ will implement those tests.

[ericgribkoff]

The decision for now was to just implement these on the client-side, because, as @ejona86 points out, Java has no mechanism to implement the CompressedRequest server-side feature of the interop test specification, which is necessary for server-side implementation of client_compressed_unary and client_compressed_streaming (in these tests, the client sends compressed/uncompressed messages and the server must verify the requests are compressed/uncompressed).

The client-side tests in PR #3211 include server_compressed_unary and server_compressed_streaming, designed to be run against a C++ server implementation that can respond appropriately to the request_compressed flag in the request messages. The Java clients cannot actually assert that the server's responses are compressed/uncompressed^[1], and grpc/grpc#11739 updates the interop test descriptions to allow for this limitation.

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[1]: Technically, the Java client could override MethodDescriptor.Marshaller#parse(InputStream stream) to do a stream instanceof KnownLength to determine if the message was received compressed (does not implement KnownLength) or uncompressed (does implement KnownLength), but this is decidedly not an intended use of the API, and it does not add enough value to the test coverage to be worth supporting this type of use.

[ericgribkoff]

Closed via #3211.