Support system scope metadata schemas

Some metadata records will adhere to schemas that are the same across all tenants. For example, Airavata will have a metadata schema for experiment instances and that schema would apply to all tenants that have Airavata experiments. So it would be useful to not have to redefine global schemas again for each tenant.

On the query side of things, I think the API will support a query that looks like this:
```
select * from system.experiment where ...
```

where `experiment` is a system scoped metadata schema.

Start with the proposed API query shape, `system.experiment`, and the distinction between system-scoped and tenant-scoped metadata schemas. Review the catalog's schema and query design before defining the implementation scope. Done should allow a global schema to apply across tenants without redefinition and support queries against it.

Written by the indexing model from the issue text.