feat: context support in protocol

[volodkevych]

Problem

We need to support chat context selection from files, folders, symbols and prompts in the protocol and chat-client. We need to support create prompt requests from chat UI.

Solution

• Extended protocol to support server providing context commands to chat UI - context commands can be nested
• Icons support was also added to quick action commands, as quick actions require icons too (extended protocol now, but actual icons will be set later)
• Extended protocol to support create prompt requests from chat UI to server

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.