[Security] Periodic testing for security against MKI, for latest Kiba…

…na commit (elastic#171859)

## Summary

In this PR the Kibana image override is introduced. 

The user will be able to decide whether the pipeline will be testing
against the code that is already deployed in QA **OR** if the project
will be created with an override from the latest commit in kibana.

This override is created in the first step of the pipeline where a new
freshly baked kibana image is built and pushed into
`docker.elastic.co/kibana-ci/kibana-serverless` with tag
`git-${BUILDKITE_COMMIT:0:12}` which is the exact commit checked out in
the current build. This is then pushed to the above mentioned registry
and as a result an image like
`docker.elastic.co/kibana-ci/kibana-serverless:git-46iuy423rn3` is being
used as the override.

In order to run the tests with the override the environment variable
`KIBANA_MKI_USE_LATEST_COMMIT=1` needs to be passed when starting the
build as shown in the following snippet.

![Screenshot 2024-01-23 at 12 49
03 PM](https://github.com/elastic/kibana/assets/55240027/273fe0f9-5dcd-403d-84ce-6c79365916da)

By default, if the environment variable is not passed, the tests will
run against the already deployed version in QA without any override.

**If only** the override flow is picked, then the Build Kibana Image
will build and push the new image. On the other hand if the environment
variable is absent, then the Build Kibana Image step will exit
immediately with exit code 0 (passed in buildkite pipeline) and proceed
to run the tests normally.


### Checklist

Delete any items that are not applicable to this PR.

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] Any UI touched in this PR is usable by keyboard only (learn more
about [keyboard accessibility](https://webaim.org/techniques/keyboard/))
- [ ] Any UI touched in this PR does not create any new axe failures
(run axe in browser:
[FF](https://addons.mozilla.org/en-US/firefox/addon/axe-devtools/),
[Chrome](https://chrome.google.com/webstore/detail/axe-web-accessibility-tes/lhdoppojpmngadmnindnejefpokejbdd?hl=en-US))
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This renders correctly on smaller devices using a responsive
layout. (You can test this [in your
browser](https://www.browserstack.com/guide/responsive-testing-on-local-server))
- [ ] This was checked for [cross-browser
compatibility](https://www.elastic.co/support/matrix#matrix_browsers)


### Risk Matrix

Delete this section if it is not applicable to this PR.

Before closing this PR, invite QA, stakeholders, and other developers to
identify risks that should be tested prior to the change/feature
release.

When forming the risk matrix, consider some of the following examples
and how they may potentially impact the change:

| Risk | Probability | Severity | Mitigation/Notes |

|---------------------------|-------------|----------|-------------------------|
| Multiple Spaces—unexpected behavior in non-default Kibana Space.
| Low | High | Integration tests will verify that all features are still
supported in non-default Kibana Space and when user switches between
spaces. |
| Multiple nodes—Elasticsearch polling might have race conditions
when multiple Kibana nodes are polling for the same tasks. | High | Low
| Tasks are idempotent, so executing them multiple times will not result
in logical error, but will degrade performance. To test for this case we
add plenty of unit tests around this logic and document manual testing
procedure. |
| Code should gracefully handle cases when feature X or plugin Y are
disabled. | Medium | High | Unit tests will verify that any feature flag
or plugin combination still results in our service operational. |
| [See more potential risk
examples](https://github.com/elastic/kibana/blob/main/RISK_MATRIX.mdx) |


### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

---------

Co-authored-by: kibanamachine <[email protected]>