Optional tagging feature: adding tags to group dbs for running queries

[marianogappa]

Use case:
Currently there's two ways to run a query in more than one db concurrently:

1. sql all to run on all dbs

2. sql dbname1 dbname2 ... to run on those specific dbs

Sometimes there's a semantic grouping for some dbs, e.g. region, owner, schema differences. Tagging dbs with these semantic differences would enable one to run e.g. sql americas ....

Constraints:

• Should be backwards-compatible.

• Tags can collide with database names: in this case, database should win.

[langbeinmovio]

@marianogappa Hi 👋