Add count attribute to TagSerializer

[jerabekjiri]

Is your feature request related to a problem? Please describe.

Old galaxy (https://old-galaxy.ansible.com/search) has a list of popular tags. Since we're reusing TagSerializer it would be useful to add this to TagSerializer in pulp_ansible.

Describe the solution you'd like
Update TagSerializer with

    count = serializers.SerializerMethodField()

    def get_count(self, obj):
        return obj.ansible_collectionversion.count()

    class Meta:
        model = Tag
        fields = ["name", "count"] 

Describe alternatives you've considered
If this doesn't work out, we can add it to galaxy_ng codebase.

[mdellweg]

sounds feasible...