Add support for capsule content counts

[vsedmik]

Description of changes

This PR adds

1. support for capsule content count endpoints added in 6.15/stream
2. a few fields that appear useful to me and API returns them already

Upstream API documentation, plugin, or feature links

https://<<SAT_FQDN>>/apidoc/v2/capsule_content.en.html

Functional demonstration

# Demonstrate functional Snapshot read in ipython
In [1]: from robottelo.hosts import Satellite
In [2]: sat = Satellite('satellite.redhat.com')
In [3]: ngc = sat.api.Capsule().search(query={'search': f'name=capsule.redhat.com'})[0]
In [4]: ngc.content_counts()
Out[4]: 
{'content_view_versions': {'4': {'repositories': {'4': {'rpm': 58,
     'erratum': 32,
     'rpm.repo_metadata_file': 1},
    '10': {'rpm': 32,
     'erratum': 4,
     'package_group': 2,
     'rpm.packagecategory': 1}},
   'cv_version_content_counts': {'rpm': 90,
    'erratum': 36,
    'package_group': 2,
    'rpm.packagecategory': 1,
    'rpm.repo_metadata_file': 1}}}}
In [5]: ngc.content_update_counts()
Out[5]: 
{'id': '0d498591-fda8-4287-b5e3-3c577de28967',
 'label': 'Actions::Katello::CapsuleContent::UpdateContentCounts',
 'pending': False,
 'action': 'Update Content Counts',
 'username': 'admin',
 'started_at': '2023-10-18 13:42:19 UTC',
 'ended_at': '2023-10-18 13:42:20 UTC',
 'duration': '0.555393',
 'state': 'stopped',
 'result': 'success',
 'progress': 1.0,
 'input': {'smart_proxy_id': 2,
  'current_request_id': '40535e72-ded4-445d-b187-5015d2a7d198',
  'current_timezone': 'UTC',
  'current_organization_id': None,
  'current_location_id': None,
  'current_user_id': 4},
 'output': {},
 'humanized': {'action': 'Update Content Counts',
  'input': [],
  'output': '',
  'errors': []},
 'cli_example': None,
 'start_at': '2023-10-18 13:42:19 UTC',
 'available_actions': {'cancellable': False, 'resumable': False}}

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (f714255) 92.15% compared to head (9b05162) 92.18%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1017      +/-   ##
==========================================
+ Coverage   92.15%   92.18%   +0.02%     
==========================================
  Files           6        6              
  Lines        3047     3057      +10     
==========================================
+ Hits         2808     2818      +10     
  Misses        239      239              

Files	Coverage Δ
nailgun/entities.py	91.33% <100.00%> (+0.03%)	⬆️

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[sambible]

lgtm