fix(client): adjust TransportSenderT (#852)

* fix(client): adjust TransportSenderT

This is trait contains `WebSocket` specific details and it's difficult to fix it properly with an extension trait
in the current design.

So this PR documents and marks it clearly that these methods are optional to implement, kind of ugly but better.

* fix build

* Update core/src/client/mod.rs

Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>

* Update core/src/client/mod.rs

Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>

* increase margin for failing test

* Update core/src/client/mod.rs

Co-authored-by: James Wilson <james@jsdw.me>

* remove optional prefix in trait

Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>
Co-authored-by: James Wilson <james@jsdw.me>