feat(bigtable/emulator): allow listening on Unix Domain Sockets

[flokli]

cbtemulator listening on unix domain sockets is much easier than trying to allocate free TCP ports, especially if many cbtemulators are run at the same time in integration tests.

This adds an additional flag, address, which has priority if it's set, rather than host:port.

NewServer already takes a laddr string, so we simply check for it to contain slashes, and if so, listen on unix, rather than TCP.

[flokli]

This can be tested as follows:

$ ./emulator --address $PWD/emulator.sock &
Cloud Bigtable emulator running on /home/flokli/dev/google-cloud-go/bigtable/cmd/emulator/emulator.sock
$ export BIGTABLE_EMULATOR_HOST=unix://$PWD/emulator.sock
$ cbt -instance instance-1 -project project-1 createtable table-1
2024/03/29 11:02:04 -creds flag unset, will use gcloud credential
$ cbt -instance instance-1 -project project-1 ls
2024/03/29 11:02:15 -creds flag unset, will use gcloud credential
table-1

No changes in the client sdk itself needed, only in the emulator.

[flokli]

conventionalcommits.org seems stuck on an old commit message, not sure how to get it unstuck.

[flokli]

I got confused over the conventionalcommits.org check - it was complaining about the PR title, not commit messages. It's green now, and the PR is rebased. Would be nice to get some feedback :-)