Add initial interop tests

tools/http2_interop/README.md

tools/http2_interop/doc.go

tools/http2_interop/frame.go

tools/http2_interop/frameheader.go

tools/http2_interop/http2interop.go

tools/http2_interop/http2interop_test.go

tools/http2_interop/ping.go

tools/http2_interop/settings.go

tools/http2_interop/unknownframe.go