fix(client): adjust TransportSenderT

[niklasad1]

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.

[lexnv]

The multiple_blocking_calls_overlap test causes the CI to fail, which is highly sensitive
to scheduling noise.

thread 'multiple_blocking_calls_overlap' panicked at 'Expected less than 60ms, got 63.3412ms'

I suggest increasing the threshold to 80ms for this case 😄

[jsdw]

I like that the default impls don't fail now; it makes much more sense to have default impls when they can safely be ignored if the implementor doesn't care about that functionality :)

For that reason, I'm not sure they need the optional_ prefix; they are optional now in the same way that any trait fn with a default impl is, so I don't think the prefix adds anything.

[niklasad1]

For that reason, I'm not sure they need the optional_ prefix; they are optional now in the same way that any trait fn with a default impl is, so I don't think the prefix adds anything.

Removed it now, I guess the documentation is clear enough now so we don't need that.
I was almost considering doing doc(hidden) to hide this from other users but I think this should be ok.