Support true binary metadata (`GRPC_ALLOW_TRUE_BINARY_METADATA`)

[edsko]

References:

• https://grpc.io/docs/guides/metadata/ (background)
• https://github.com/grpc/proposal/blob/7c05212d14f4abef5f74f71695f95ba8dd3f7dd3/G1-true-binary-metadata.md (spec)

The C++ reference implementation in the gRPC interop tests enables this in the initial HTTP2 SETTINGS frame (Wireshark shows this as Unknown (65027) : 1; 65027 == 0xFE03). I'm not yet sure if this is causing problems for the test suite.