feat(audit) add initiator field to relevant events (almost all of them)

This patch adds initiator field to most of the events. Initiator is
added to members of Op enum which do not contain it already (e g owner
in CreateUser serves as initiator). To me it is reasonable to remove
as_user from CaS/ACL checks because it matches initiator. But I decided
not to do this just yet.

Additionally I've mande small cleanup to UserMetadata to avoid need for
intermediate step in decoding when we first fetch tuple and then decode
it into model struct.

Close https://git.picodata.io/picodata/picodata/picodata/-/issues/433