feat: initial OpenFGA `v2beta1` protobuf API development

[jon-whit]

Description

Start the development branch for the v2beta1 protobuf API definitions for OpenFGA. This is to get things started with that new API development effort as we go into the new year.

As we work through these changes in the scope of this PR, lets try to focus on a few guiding principles:

1. Well typed and strongly typed protobuf API contract - prefer objects over string encoded entities where you can (e.g. Object instead of string object)

2. Separation of input types vs output types and a clear definition of required vs non-required fields within them- this really helps other development in our tooling such as in the SDKs etc..

3. Think about the gRPC experience, not just the HTTP mapping - the protobuf API is the definitive source of truth. The HTTP API is derived off of it. When in doubt, the protobuf API should take precedence.

4. Give a shit about naming things - if you've had a beef with the API names, speak up and fight for the cause 😄

5. Encourage strong alignment with buf lint lint style guidelines so that we have a standard - we shouldn't have to make exceptions regularly for our protobuf API linting behavior. Stick to the community standard of using buf lint and the default rules for it.

6. Consider Google API Improvement Proposals (AIPs) as a good starting point for inspiration behind certain API changes and design - we don't have to copy Google API guidelines, but they've clearly given a lot of API experience some thought and since they are the primary driving force behind gRPC and RPC APIs in general, their guidelines are a good reference point for ideas of achieving certain design goals. In particular, consider the AIPs around standard methods.

A few notable changes to get started:

• The new API has been designed with an emphasis on a strongly typed protobuf API. For example, instead of string object fields we have explicit Object object fields.

• User fields have been renamed to Subject to avoid confusion with the often defined type name user. Subject is a strongly typed entity that can be one_of Object (a direct object), TypedWildcard (to represent the public wildcard), or a SubjectSet (to represent the set of subjects that expand to one or more concrete objects). A SubjectSet models Usersets in Zanzibar nomenclature.

• TupleKey has been more aptly named RelationshipTuple - this coincides with our official documentation Concepts better (see Relationship Tuple)

• Userset, which previously represented a relation rewrite rule, has been refactored to a more aptly named RelationRewrite and the model of it is much more uniform.

• The relations defined in an AuthorizationModel has changed from a direct Userset rewrite field to a map of Relation structures map<string, Relation>. This will make things more uniform by far in all application code that deals with an AuthorizationModel structure and will more easily allow for indexing into the model's relation map to grab a particular relation.

• Relation now uniformly represents a relation definition, and the constructs associated with it such as the type restrictions that apply to it etc..

• The ReadChanges API has been renamed to WatchChanges and has been relocated to an auxillary gRPC service definition WatchService. This will allow for a separate watch server to implement the WatchService and allow that service to be hosted and scaled independently of the main OpenFGAService which services Checks, ListObjects, etc.. The ability to serve and scale this service independently will be more important especially as we start building other features around changelog metadata such as the indexer and ExpandedWatchChanges endpoints etc.. These workloads will be different and need not be served from the same server in general..

References

Review Checklist

• [ ] I have clicked on "allow edits by maintainers".
• [ ] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• [ ] The correct base branch is being used, if not main
• [ ] I have added tests to validate that the change in functionality is working as expected

[jon-whit]

• [ ] todo: add protobuf doc to every protobuf message and field as appropriate.
• [ ] todo: add required field specifications and other protobuf annotations for OpenAPI generation