sql: organize / track better which vtables are implemented

[RichardJCai]

Currently, we have an allTableNames field which we try to list all the name of all vtables. This is useful for providing error messages when a user tries to access a vtable that we know is unimplemented and have in the allTableNames list. The big issue with this is that it is all manual and does not correspond to our actual vtable definitions.

type virtualSchema struct {
	name           string
	allTableNames  map[string]struct{}
	tableDefs      map[descpb.ID]virtualSchemaDef
	tableValidator func(*descpb.TableDescriptor) error // optional
	// Some virtual tables can be used if there is no current database set; others can't.
	validWithNoDatabaseContext bool
	// Some virtual schemas (like pg_catalog) contain types that we can resolve.
	containsTypes bool
}

Adding a name is manual and prone to error.

Some options we have moving forward with this is to

1. Remove the field (it is only used for error logging), we have a separate pg_catalog_expected_diffs.json file now which we can use to track.
2. Make it correspond to the the actual tables / table ids somehow so it's less manual and we know if we implement it or not. Can also add tests to check the name against if the table desc is created.

In general, I think it would be nice to be able to track the status of vtables and see their implementations all in one place. Using a map that maps the table name to a struct holding it's definition, oid and status could be nice / an option?

We should choose one reliable source of truth on the status of different vtables.