implement unix domain socket support

[snorkelopstesting4-web]

Recreated from original PR: borgbackup/borg#7615

Server side (listening side):

borg serve --socket=/path/to/socket
or
borg serve --socket  # finds socket at `get_runtime_dir()/borg.sock`

Client:

borg --socket=/path/to/socket --repo=socket:///path/to/repo rcreate ...
or
borg [--socket] --repo socket:///path/to/repo create ...  # finds socket at `get_runtime_dir()/borg.sock`

BORG_RUNTIME_DIR env var can be used to override BORG_BASE_DIR and platformdirs.user_runtime_dir, as usual.

fixes #6183.