Add thumbnail endpoints

[tdstein]

Content Thumbnail API

v0.6.0 Proposal

from posit import connect

client = connect.Client()
content = client.content.get(...)

# GET https://connect.company.com/content/{guid}/__thumbnail__
# Cache the thumbnail
thumbnail = content.thumbnail

# PUT /v1/content/{guid}/thumbnail
thumbnail = content.create_thumbnail(...)

# Remove the cached thumbnail.
# The next get operation reads from the server
content.reset_thumbnail()

# Same as....
# >>> content.reset_thumbnail()
# >>> thumbnail = content.thumbnail
thumbnail = content.reload_thumbnail()

# DELETE /v1/content/{guid}/thumbnail
thumbnail.destroy()