Support for enabling database extensions

[arturo-seijas]

There's currently no way to enable database extensions as the connection user. The database charm will not be aware of the user the related application is requesting, so setting up the extensions through the database charm configuration is not an option.

IMHO the relation library should support enabling this extensions. Otherwise, an SQL client library would need to be included to execute the corresponding SQL statements. One possible solution would be for the library to support this SQL execution.

[mthaddon]

Note that this is something that's currently supported by https://github.com/canonical/ops-lib-pgsql - obviously the implementation doesn't need to be the same, but some charms using this library are relying on being able to do this currently.