client: encode the authority by default

[anicr7]

Currently the endpoint is returned as the authority unless it can be determined by the scheme or from the dial or credentials. In case the endpoint has multiple slashes, this can be invalid. To handle this use case encode the authority by default.

In the future this can be subsumed by adding per resolver builder method that can provide a more sophisticated logic for determining authority.

@dfawley

PS: This is a rollforward of #6318 with slightly different logic to handle authority specific delims rather than piggy backing on the path escape.

RELEASE NOTES:

• client: percent-encode the default authority for the channel