Add default url redirect to url maps

[modular-magician]

Fixes hashicorp/terraform-provider-google#6081

A few things that came up while developing this:

• RegionTargetHttpProxies are currently in the google-beta terraform provider even though theses resources seem to be GA now: https://www.terraform.io/docs/providers/google/r/compute_region_target_http_proxy.html / https://cloud.google.com/compute/docs/reference/rest/v1/regionTargetHttpProxies -> I did not change the example yet.
• urlRedirect parameter seems to be slightly out-of-sync: #3378
• Not sure if/how to write tests for this new feature?
• Not sure if the google_compute_target_http_proxy resource is the best place for the example?
• Also the API docs state: "If defaultUrlRedirect is specified, defaultService or defaultRouteAction must not be set." (How) is such mutual exclusivity modeled in magic-modules?

Release Note Template for Downstream PRs (will be copied)

compute: Added support for default URL redirects to `google_compute_url_map`

compute: Added support for default URL redirects to `google_compute_region_url_map`

Derived from GoogleCloudPlatform/magic-modules#3379