fix: fail validation on create connector if connector already exists

[vcrfxia]

Description

Quick follow-up to #8999 (specifically, this discussion): for CREATE CONNECTOR statements that do not include IF NOT EXISTS in the statement, these statements will fail if a connector with the same name already exists. Currently, this failure happens during ksql's execute phase. Ideally, the failure would occur during ksql's validation phase, in order to fail fast so that no statements (in a multi-statement request) are executed. This PR adds a check for this case during validation accordingly.

A consequence of this PR is that users are now required to have permissions to list existing connectors in order to issue CREATE CONNECTOR statements (even for statements that do not include IF NOT EXISTS). We believe this is inline with the desired security model for ksql, and are OK with this consequence.

Testing done

Unit + integration.

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[Gerrrr]

Thank you for fixing this issue!