fix: Define values of `altas_gcp_project_id` and `atlas_vpc_name` in network peering by fetching associated container

Description

Link to any related issue(s): https://github.com/mongodb/terraform-provider-mongodbatlas/issues/2308 (CLOUDP-250959)

During the SDK migration (https://github.com/mongodb/terraform-provider-mongodbatlas/pull/2245) we changed the way we populate the computed values of altas_gcp_project_id and atlas_vpc_name. This breaks our example for creating a gcp peering network as it makes use of these computed values related to gcp.

These values were (previous to 1.16.1) fetched by obtaining the associated container (reference) and are now using values provided directly from the peering network response.

Example network_peering state 1.16.0

"attributes": {
            "atlas_gcp_project_id": "p-c1zgxlsyh45xj2dwadq0nlbe",
            "atlas_vpc_name": "nt-66564bd12d057908c6243c07-ql63a9ab",
            "gcp_project_id": "ofuscating",
            "network_name": "randomName",
            ...
          },

Example network_peering state 1.16.1

"attributes": {
            "atlas_gcp_project_id": "ofuscating",
            "atlas_vpc_name": "randomName",
            "gcp_project_id": "ofuscating",
            "network_name": "randomName",
            ...
          },

Example network_peering state after fix

"attributes": {
            "atlas_gcp_project_id": "p-45nlqzaiphqffrdbmzjk3mvy",
            "atlas_vpc_name": "nt-66564bd12d057908c62e3c07-0o71uun2",
            "gcp_project_id": "ofuscating",
            "network_name": "randomName",
            ...
          },

Type of change:

[x] Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
[ ] New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
[ ] This change requires a documentation update
[ ] Documentation fix/enhancement

Required Checklist:

[x] I have signed the MongoDB CLA
[x] I have read the contributing guides
[x] I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
[x] I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
[x] I have added any necessary documentation (if appropriate)
[x] I have run make fmt and formatted my code
[x] If changes include deprecations or removals, I defined an isolated PR with a relevant title as it will be used in the auto-generated changelog.
[x] If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

mongodb / terraform-provider-mongodbatlas

fix: Define values of `altas_gcp_project_id` and `atlas_vpc_name` in network peering by fetching associated container #2315