* git: support client kwargs for auth in remote methods

* tests: use docker for git/ssh server

* move dulwich backend into subdir

* add base sync<->async support

* dulwich: add initial asyncssh SSH vendor

* dulwich: use asyncssh as the default SSH vendor

* add test for git over ssh

* test read (fetch) over ssh

* ci: re-enable ssh tests

temporary workaround for #6788, #6789

* move async stubs into dvc.scm.asyn

* tests: use sshfs and not dvc.fs.ssh

* allow explicit 0-length read in asyncssh vendor