[api][handlers] Stub isolation_segment related endpoints

[benjaminguttmann-avtq]

Is there a related GitHub Issue?

#4244

What is this change about?

As discussed in the bi-weekly Working group meeting I stub the return of the iso-segment related endpoints of org and space.

Does this PR introduce a breaking change?

No

Acceptance Steps

cf create-org bg-test
cf curl /v3/organizations/$(cf org bg-test --guid)/relationships/default_isolation_segment

cf t -o bg-test
cf create-space bg-test

 cf curl /v3/spaces/$(cf org bg-test --guid)/relationships/isolation_segment

Tag your pair, your PM, and/or team

That is the first part on the way to make cf org and cf space work.

[danail-branekov]

Looks good @benjaminguttmann-avtq

I would suggest to also add e2e tests that invoke the new endpoints and assert an OK response to make sure the new endpoints are properly wired.