Add docker_manifests endpoint to repositories

[vsedmik]

Description of changes

We are missing docker_manifests endpoint so we can't read the manifests in a docker repository.

Upstream API documentation, plugin, or feature links

https://satellite.redhat.com/apidoc/v2/docker_manifests.en.html

Functional demonstration

Example:

from robottelo.hosts import Satellite
sat = Satellite('satellite.redhat.com')
sat.api.Repository(id=55).docker_manifests()
Out[4]: 
{'total': 1,
 'subtotal': 1,
 'selectable': 1,
 'page': 1,
 'per_page': 20,
 'error': None,
 'search': None,
 'sort': {'by': None, 'order': None},
 'results': [{'id': 1672,
   'schema_version': 2,
   'digest': 'sha256:23973d32066ae1cb15ce3cef7fe66a8aa87038936b6c828cc5c129d46a3a9233',
   'manifest_type': 'image',
   'annotations': {'org.opencontainers.image.base.name': 'docker.io/library/alpine:3.18',
    'org.opencontainers.image.base.digest': 'sha256:695ae78b4957fef4e53adc51febd07f5401eb36fcd80fff3e5107a2b4aa42ace'},
   'labels': {'containers.bootc': '1', 'io.buildah.version': '1.33.5'},
   'is_bootable': True,
   'is_flatpak': False,
   'tags': [],
   'manifest_lists': [{'id': 197,
     'digest': 'sha256:6a4a6aded92e720d91ec9bfdb0bf80a37846ac837be652f4d87140a46dc4ba71',
     'schema_version': 2,
     'manifest_type': 'list'}]}]}