[UI Feature] Display user identity in the UI

### Motivation: Why do you think this is important?

The user identity of execution (and ideally also workflow registration) user should be available in the UI. This would enable good collaboration because one could easily see which workflows and which executions belong to which users.

### Goal: What should the final outcome look like, ideally?

* Besides the JWT subject claim (which is the current "user" as in `ExecutionIdentity`), one should also store a real user name in the postgres backend.
* It should be possible to filter executions by user (now it's just executions belonging to the current user or all executions)
* User info would be also a special column in the executions overview.
* User info would be also linked to versions (registrations).

### Propose: Link/Inline OR Additional context

_No response_

### Are you sure this issue hasn't been raised already?

- [x] Yes

### Have you read the Code of Conduct?

- [x] Yes