Support IF EXISTS keyword on DROP CONNECTOR

[spena]

Is your feature request related to a problem? Please describe.
Support the IF EXISTS keyword on `DROP CONNECTOR. It is useful when executing repeatable SQL scripts that replaces connectors like:

DROP CONNECTOR IF EXISTS <connector>;
CREATE CONNECTOR <connector> ...;

Describe the solution you'd like
Drop the connector only if it exists. No-op if it doesn't.

Describe alternatives you've considered
No alternatives.