add readBulkRelationships-API

[kartikaysaxena]

Related authzed/spicedb#2160

This specifies the skeletal of the API

[josephschorr]

@kartikaysaxena Thank you for the contribution. A few questions/comments.

1. Is the goal here to have fully independent read relationship calls? If so, as gRPC is already parallelized, there is little-to-no benefit of having a combined bulk read method
2. If the goal is to not have the read be fully independent, then wouldn't the cursor need to apply to the entire operation, rather than each individual read?

[kartikaysaxena]

Hey @josephschorr implemented the cursor for the entire ReadBulkRelationships call not for the individual reads, would also be raising some PRs for implementing this in spiceDB.