Prohibit assigning topology-based delivery services to caches

[rawlinp]

What does this PR (Pull Request) do?

Update the following endpoints to prevent assigning caches to delivery services that have a topology assigned:

POST servers/{id}/deliveryservices
POST deliveryservices/{xml_id}/servers
POST deliveryserviceserver
POST cachegroups/{id}/deliveryservices

• This PR is related to Adjust cachegroup parentage and deliveryservice-server assignment endpoints for Topologies #4572 but does not complete it.

Which Traffic Control components are affected by this PR?

• Traffic Control Client (Go)
• Traffic Ops

What is the best way to verify this PR?

Run the unit tests. Run the TO API tests (negative tests were added for all 4 routes). Verify they all pass.

The following criteria are ALL met by this PR

• This PR includes tests
• New validation, no docs changes necessary
• This PR includes an update to CHANGELOG.md
• This PR includes any and all required license headers
• This PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[zrhoffman]

API tests pass, unit tests pass, code is formatted correctly, and manual testing confirms you can't assign a topology-based delivery service to an edge cache bug you can assign it an origin.

Just a couple of comments about error messages. Besides that, this looks good.

[zrhoffman]

LGTM