Make schema validation and collaboration easier

[corkrean]

Problem statement: Presently, users with large amounts of test relationships and assertions have to create a single large .yaml file for validation. This single large .yaml file can make it difficult for multiple teams to contribute test relationships and assertions.

If teams want to modularize their test relationships and assertions, they have to create a system that will combine their schema, test relationships, and assertions from multiple files into a single file. This takes time to build and maintain.

Potential Solution zed validate could take a more modular approach by reading a single schema file, followed by a variable amount of files containing test relationships and assertions.

related to #497

[josephschorr]

Related: https://github.com/authzed/spicedb/issues/1437