Policy API: should be able to update an existing attribute value's members

[jakedoublev]

Background

https://github.com/opentdf/platform/issues/105 helped add functionality to create Attribute Values with Members. However, there is a need to update Attribute Values that already exist. From there, a Client can rely on the update API to add, remove, or replace members of a value.

Acceptance Criteria

One of the following two options is employed:

1. UpdateAttributeValues RPC and data layer allow complete replacement of members by a Client
   • update of value members is considered a full replacement of whatever member values previously existed
   • the client is responsible for deciphering what the final replaced members should be
2. UpdateAttributeValues RPC and data layer rely on a behavior enum construct similar to metadata to allow extending or full replacement of value members

In either implementation, the following:

• multiple roundtrips to the DB are considered acceptable due to the comparative infrequency of updates relative to reads
• thorough integration tests are written to prove the behavior is as expected

[jakedoublev]

This should not be done while Product clarifies the story and concepts around Members/GroupBy.

[jakedoublev]

Won't do.

[jakedoublev]

https://github.com/opentdf/platform/issues/984