Handle `push --delete` command to delete remote refs

In addition to the sha.bundle file, a repo.zip file gets created and uploaded into the same remote refs path.

Example use-case: Use a git-remote-s3 repo as S3 Source in AWS CodePipeline by pointing to the repo.zip file which will be updated on each push.

When the remote URI scheme is 's3://' then only the sha.bundle is uploaded.
When 's3+zip://' is used as URI scheme, then the additional zip archive will be created and pushed as well.

Only Accept s3 and s3+zip in regex match

Adjust cmd_push delete logic to work with s3+zip uri scheme and an additional file next to the bundle

Use Enum instead of magic strings for uri scheme

Add test for wrong uri scheme s3+foo

Add test cases for cmd_push delete with s3+zip as uri scheme

When tests use 'main' branch, local pytest runs can differ from pipeline runs.

For example: There is no git repo when pytest runs in the pipeline.
But locally it runs inside the git-remote-s3 repo.
If mocking of a git command wasn't done properly, local tests pass because a git repo with a main branch exist.
They then fail in the pipeline with an error like: 'fatal: not a valid object name: refs/heads/main'

By moving away from a branch name that exists locally, the same behavior as in a pipeline run is enforced and problems easier to discover.

feat: Create and upload zip archive on git push

doc: add instructions for submodules

Correct handling of multilevel paths in S3 prefixes