Stream EOF support

[cthulhu-rider]

Currently we don't have any mention of a graceful end to the stream. Go gRPC implementation returns EOF on client/server side if server/client closed the stream via corresponding library functionality.

Do we want to unify this behavior on protocol definition level? Possible solutions:

1. Require to finish stream with empty request|response body: this will support the completion of the stream at a purely protocol level, and not be tied to the implementation;
2. Require to finish stream with EOF-like status code: alternative way of 2.

[cthulhu-rider]

i thought that the synonymous problem could be lack of unknown RPC: Protocol Buffers v3 doesnt contain functionality for this. Hence current NeoFS API protocol doesnt specify behavior when calling, for example, neo.fs.v2.object.ObjectService/MissingRPC method. Some dedicated status message would help

https://datatracker.ietf.org/doc/html/rfc2616#section-10.5.2

[cthulhu-rider]

consider specific status code similar to 206 Partial Content. Then all non-fin messages could carry this code while the fin one 0 OK meaning EOF

but be careful, backward compatibility never sleeps