Add Go implementation of GET, POST federations/{id}/federation_resolvers

[lactose]

What does this PR (Pull Request) do?

This is a rewrite of the perl endpoints GET,POST for /federations/{id}/federation_resolvers

• This PR fixes Rewrite /federations/{{id}}/federation_resolvers to Go #3795

Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

Run the added API test against both the Perl and Go backends.

You can also test this in TP by adding federation resolvers to federations and ensuring that they show up in the UI.

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR 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)

[asf-ci]

Can one of the admins verify this patch?

[mitchell852]

@lactose - i'm seeing a couple of failing API tests:

=== RUN   TestCDNFederations/successfully_get_federation_federation_resolvers_for_a_federation_with_some
    --- FAIL: TestCDNFederations/successfully_get_federation_federation_resolvers_for_a_federation_with_some (0.02s)
        cdnfederations_test.go:222: expected federation of ID 19 to have associated federation resolvers, but had 0

=== RUN   TestFederationFederationResolvers/Fail_to_assign_federation_resolver_to_a_federation_when_federation_does_not_exist
    --- FAIL: TestFederationFederationResolvers/Fail_to_assign_federation_resolver_to_a_federation_when_federation_does_not_exist (0.01s)
        cdnfederations_test.go:186: error: expected error result 404 Not Found[404] - Error requesting Traffic Ops https://localhost:8443/api/1.4/federations/-1/federation_resolvers {"alerts":[{"text":"'-1': no such Federation","level":"error"}]}
            , want: Resource not found

[lactose]

@mitchell852 tests are now working

[mitchell852]

@lactose - i'm seeing a difference between the Perl's GET behavior and the Go's GET behavior:

Perl:

GET api/1.4/federations/1/federation_resolvers where no fed resolvers are assigned to the federation:

{"response":[]}

Go:

GET api/1.4/federations/1/federation_resolvers where no fed resolvers are assigned to the federation:

{"response":null}

The POST, however, looks fine.

[lactose]

@mitchell852 pushed an update

[mitchell852]

Looks like all the PR comments have been addressed and the behavior emulates that of the Perl endpoints. All unit tests and API tests pass as well as well as a manual test in TP.