external connection: event log CREATE and DROP

Creation, alteration and deletion of an external connection could explain why various processes in a cluster suddenly change behavior -- dropping the connection could explain why backups fail, or altering could explain why changefeeds suddenly change their outbound network characteristics -- meaning that being able to monitor, analyze and correlate these events could be important to an operator. Conformity with expectations of other DDL and DDL-like statements that emit such logs also indicates these statements should emit event logs.

Jira issue: CRDB-35315