Add ability to run queries over multiple databases at once

[aeisenberg]

Some enterprise customers have asked for this capability. Even though it's useful, perhaps they should be using code-scanning instead.

[shati-patel]

This will (eventually) become useful for query console work, so I'll start experimenting a bit!

Some options discussed earlier:

• Try adding a new command to "run query on multiple databases" and then use a multi-select list of databases to run on.
• Could also try adding a command from the Databases UI to "run query on selected databases" and then ask user to choose a query.