Add ws/robust, an alternative to ws_manager

[abrkn]

This alternative to ws_manager with some improvements:

• The stream is not coupled to the InfoClient
• Subscriptions management is separated from the stream
• Subscriptions are dropped when the token is dropped
• The stream can be closed and does not leave dangling futures

It does not implement connection retries since this should be built on top as it requires informing subscribers that the stream is interrupted and the client is missing messages.

TODO

• [x] Helper on Subs to create a Stream and Subs with one fn call

[abrkn]

This is my first time writing a Rust lib. Any feedback would be welcome!

I'll be testing the lib myself in the coming weeks and expect lots of changes based on that, hence leaving this PR as a draft.