Description

This change reintroduces the UpsertGroup RPC functionality for use by the refreshExternalGroupMembershipsByEntityID function. Users were unable to log in or renew their tokens if their entity's external group association changed. This issue stemmed from performance standby nodes being unable to write group updates to storage. To fix this regression, we’ve restored the RPC functionality, allowing performance standbys to forward group update logic to the active node.

ENT PR: https://github.com/hashicorp/vault-enterprise/pull/7716

TODO only if you're a HashiCorp employee

• Backport Labels: If this fix needs to be backported, use the appropriate backport/ label that matches the desired release branch. Note that in the CE repo, the latest release branch will look like backport/x.x.x, but older release branches will be backport/ent/x.x.x+ent.
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• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
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