Add support for finding Content belong to a Group

[tdstein]

Add support for retrieving Content for a Group. This is supported via the v1/experimental/groups/:group_guid/content API. See https://docs.posit.co/connect/cookbook/content-access-controls/auditing-group-access-to-content/

This should be implemented as an attribute off of the Group resource and resemble the following pseudo-code:

from posit import connect

client = connect.Client()
group = client.groups.find_by(name="example-group")
assert group
content: List[Content] = group.content