Interop test: Python and C++ client, grapesy server

[edsko]

Status after this PR:

Legend:

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

grapesy as server

Test	Python	C++
cancel_after_begin	✅	❔
cancel_after_first_response	✅	✅
client_compressed_streaming	❔	✅
client_compressed_unary	❔	✅
client_streaming	✅	❔
custom_metadata	✅	✅
empty_stream	✅	❔
empty_unary	✅	❔
large_unary	✅	✅
orca_oob	❔	❓
orca_per_rpc	❔	❓
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	✅	❔