C++: add missing implementations for client advertised schema/encoding

[paulsohn]

Changelog

C++ server/client: add missing implementations for client advertised schema / schema encoding

Docs

• Issue C++ type definition doesn't contain schemaEncoding field #875 (Initial discussions are here)
• Liner Ticket FG-9447 (though I can't access to it)

Description

Although foxglove ws protocol allow client to advertise complete schema definition string as well as schema name, current C++ implementation lacks such fields. Particularly:

• ClientAdvertisement struct lacks schemaEncoding field,
• ClientAdvertisement struct has schema field, but it is never populated (websocket server does not read schema or schema encoding to json)
• websocket client is does not serialize schema or schema encoding to json

This PR resolves above problems, and additionally since this is breaking change from unimplemented to implemented, types are decided as following:

• schema field type is changed from std::vector<uint8_t> to std::optional<std::string>, as schema fields are passed in as json quoted string, and it's optional
• schemaEncoding field type is determined as std::optional<std::string>, the same reason for schema.

NOT TESTED YET. I plan to integrate this to my module to verify if this really works, but it would take some time.

Fixes: #875
Fixes: FG-9447

[defunctzombie]

@paulsohn Thanks for the proposed fix here - let us know when this is ready for review!

[paulsohn]

@defunctzombie Sure.
It is a draft because I got no tests but I don't think further changes might be needed, so feel free to test this as it is (I will do my tests in next week)