google_artifact_registry_repository: Fix perma-diff in maven_config

[modular-magician]

The maven_config field is immutable, but also optional. This PR adds a diff suppress to equate missing, empty and default configs.

Example:

resource "google_artifact_registry_repository" "repo" {
  project       = local.project_id
  repository_id = "test-maven-repo"
  format        = "MAVEN"
  location      = "us"
  description   = "Test Maven Repository"
}

resource "google_artifact_registry_repository" "repo" {
  project       = local.project_id
  repository_id = "test-maven-repo"
  format        = "MAVEN"
  location      = "us"
  description   = "Test Maven Repository"
  maven_config { }
}

resource "google_artifact_registry_repository" "repo" {
  project       = local.project_id
  repository_id = "test-maven-repo"
  format        = "MAVEN"
  location      = "us"
  description   = "Test Maven Repository"
  maven_config {
    allow_snapshot_overwrites = false
  }
}

These should all represent the same configuration, but instead cause perma-diffs when moving from one to any other.

Release Note Template for Downstream PRs (will be copied)

artifactregistry: fixed perma-diff in `google_artifact_registry_repository`

b/376134092

Derived from GoogleCloudPlatform/magic-modules#12190