Another go at #12341. LKB-2497
We now only need 1 connect mechanism (and 1 more for testing) which
saves us some code and complexity. We should be able to remove the final
connect mechanism when we create a separate worker task for
pglb->compute connections - either via QUIC streams or via in-memory
channels.
This also now ensures that connect_once always returns a ConnectionError
type - something simple enough we can probably define a serialisation
for in pglb.
* I've abstracted connect_to_compute to always use TcpMechanism and the
ProxyConfig.
* I've abstracted connect_to_compute_and_auth to perform authentication,
managing any retries for stale computes
* I had to introduce a separate `managed` function for taking ownership
of the compute connection into the Client/Connection pair
8daebb6ed4