Enable tagging models and stores

[aaguiarz]

• Stores have a model name that is not unique, which makes it difficult to use them to identify a store.
• Models have an ID that OpenFGA generates, which makes it difficult to link to other artifacts like Github commits, internal product versions, etc.

There are several scenarios where having other unique identifiers or additional tags for models and stores would be very useful.

A possible implementation option is described openfga/rfcs#27