As a user a can remove signatures from a repo

[ipanova]

enable signature removal form a push repo by providing :

1. signature_digest which as a result will remove a single signature from the repo
2. manifest_digest which as a result will remove all its signatures present in the repo
3. signature_key_id which as a result will remove all signatures that were signed with the mentioned key

1 and 2 can be added later upon request.

w/r/t to sync repos, it is possible to remove signature(s) by providing the href(s) to the content unit(s) and use the /remove endpoint the code path still needs updates here https://github.com/pulp/pulp_container/blob/signing/pulp_container/app/tasks/recursive_remove.py#L116 in case signature content_href was specified