This PR populates the existing `kibana.alert.workflow_user` field in the
alerts-as-data mappings with the `profile_uid` of the last user to
modify the status of the alert. It also adds a new field,
`kibana.alert.workflow_status_updated_at`, to track the last time the
workflow status was updated and populates it with a timestamp.
Similar to the alert assignment PR, `workflow_user` renders in the table
with a user avatar instead of the raw `profile_uid` value stored in the
alert. The filter in/out buttons on the row cell automatically add a
filter that uses the raw value so that filtering works correctly.
Due to limitations of Kibana's user profile implementation,
`workflow_user` is only populated if a user changes the alert status
using the alert status route (`POST
/api/detection_engine/signals/status`) within an interactive session,
i.e. logs in rather than passes credentials with each API request
([related issue](https://github.com/elastic/kibana/issues/167459)).
## Alerts table

## Alert details

### Checklist
- [ ] Functional changes are hidden behind a feature flag. If not
hidden, the PR explains why these changes are being implemented in a
long-living feature branch.
- [x] Functional changes are covered with a test plan and automated
tests.
- [x] Stability of new and changed tests is verified using the [Flaky
Test Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner).
- Flaky test run:
https://buildkite.com/elastic/kibana-flaky-test-suite-runner/builds/4130
- [ ] Comprehensive manual testing is done by two engineers: the PR
author and one of the PR reviewers. Changes are tested in both ESS and
Serverless.
- [x] Mapping changes are accompanied by a technical design document. It
can be a GitHub issue or an RFC explaining the changes. The design
document is shared with and approved by the appropriate teams and
individual stakeholders.
- https://github.com/elastic/security-team/issues/4820
- [x] Functional changes are communicated to the Docs team. A ticket or
PR is opened in https://github.com/elastic/security-docs. The following
information is included: any feature flags used, affected environments
(Serverless, ESS, or both).
- https://github.com/elastic/security-docs/issues/4325