SNI: Dynamically adjusting based on request headers

Today, SNI gets computed before starting execution based on the `authority` part of the uri(s) specified in the `uri` or `multi-target-endpoint`s. After that things are statically set up with respect to SNI.

However, it is possible that the `authority` dynamically changes throughout execution when a request source is used.
In this flow a mismatch arises between the SNI we set up and the actual authority used in requests, which seems suboptimal.

Filing this as a placeholder, I plan to provide a complete example which reproduces the issue in a follow up.