git: use asyncssh as dulwich SSH vendor (#6797)

* 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

.github/workflows/tests.yaml

@@ -109,7 +109,7 @@ jobs:
       run: >-
         python -m tests -n=4
         --cov-report=xml --cov-report=term
-        ${{ env.extra_test_args }}
+        --enable-ssh ${{ env.extra_test_args }}
     - name: upload coverage report
       uses: codecov/[email protected]
       with: