Modify API to make explorations aware of their associated schema

[seancolsen]

Context

1. In the UI, each exploration is associated with one schema by virtue of being presented "within" a schema.
2. In Mathesar 0.1.7, that association was handled by virtue of each Exploration referring to a Table which then referred to a Schema. All of those associations were contained within the internal database.
3. Mathesar 0.2 onwards no longer has that Table model. Each exploration weakly references a table oid in the user database. And we need to look inside the user database to see what schema that table lives within.
4. The change in data model presents a small problem, given our current UX.
5. At some meeting a few months ago we had decided to solve this problem within the front end. The front end would fetch all the explorations for a database and then filter them out.
6. However, now that I'm in the code I can see that taking that approach will require significant front end refactoring. Pavish and I had a call about this today and decided that it would be best to solve this problem in the service layer instead of in the front end.

Changes needed

Here's what we'd like:

• explorations.list

  • Add an optional schema_oid parameter to filter the list of explorations by schema.

  • Modify return value to include a schema_oid property.

• explorations.get

  • Modify return value to include a schema_oid property.

• explorations.add

  • Modify return value to include a schema_oid property.

To handle these changes, the service layer would need to call out to the user database for these two RPC methods.