Implement most client interop tests

[edsko]

Still to do: cancel_after_begin, cancel_after_first_message, and timeout_on_sleeping_server.

[edsko]

The status after this PR is

• ✅ passed
• ❌ failed
• ❓ unknown (test not run or not yet implemented in grapesy test server)
• ❔ not supported by the reference

Test	Python	C++	Go	Java
cancel_after_begin	❓	❓	❓	❓
cancel_after_first_response	❓	❓	❓	❓
client_compressed_streaming	❔	✅	❔	❔
client_compressed_unary	❔	✅	❔	❔
client_streaming	✅	✅	✅	✅
custom_metadata	✅	✅	❌	✅
empty_stream	✅	✅	✅	✅
empty_unary	✅	✅	✅	✅
large_unary	✅	✅	✅	✅
ping_pong	✅	✅	✅	✅
server_compressed_streaming	❔	✅	❔	❔
server_compressed_unary	❔	✅	❔	✅
server_streaming	✅	✅	✅	✅
special_status_message	✅	✅	✅	✅
status_code_and_message	✅	✅	✅	✅
timeout_on_sleeping_server	❓	❓	❓	❓
unimplemented_method	✅	✅	✅	✅
unimplemented_service	✅	✅	✅	✅

The custom_metadata failure for Go is due to https://github.com/well-typed/grapesy/issues/93.

When running in automatic rather than manual mode, the tests sometimes fail non-deterministically with HandshakeFailed. I don't yet know what that is caused by.