connection: clarify `transport`

connection: clarify `transport`

The `transport` to use for a transfer, e.g. TCP/QUIC/UNIX/UDP, is
initially selected by options and protocol used. This is set at the
`struct connectdata` as `transport` member.

During connection establishment, this transport may change due to
Alt-Svc or Happy-Eyeballing. Most common is the switch from TCP to QUIC.

Rename the connection member to `transport_wanted` and add a way to
query the connection for the transport in use via a new connection
filter query.

The filter query can also be used in the happy eyeballing attempts when
code needs to know which transport is used by the "filter below". This
happens in wolfssl initialization, as one example.

Closes #17923