[Shareable Dashboards] Make image embeddable shareable

[cqliu1]

Summary

Note: this PR is merging into this Feature Branch: #168166
This will be merged into main now.
Blocked by #171461.
Closes #170770.

This enables sharing for image embeddables and file saved objects.

Copy to spaces and import/export of a file saved object doesn't currently work and is not within the scope of this PR.

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests 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)
• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

Risk Matrix

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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

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[vadimkibana]

Currently files don't support export/import, as besides exporting the metadata saved object we would also need to copy the file blob contents. From my understanding it can be achieved, but we would need to implement that first.

[cqliu1]

I'm going to set importableAndExportable: false and won't be tackling support for import/export as part of this PR/sharing dashboards feature. After testing, it seems the combination of hidden: true and importableAndExportable: false allows files to be shared but prevents file objects from appearing in the Saved Object Management page.

[nickpeihl]

Since file objects are hidden from the Saved Object Management page, there is no way in the UI to share files to other Spaces. To resolve this we need to add the "Spaces" column to the filesManagement page. I have a PoC PR that shows how we could eventually implement that feature. But I don't see why that should block this PR going into the feature branch.

[elasticmachine]

Pinging @elastic/appex-sharedux (Team:SharedUX)

[rshen91]

Shared ux changes LGTM - code review only

[nickpeihl]

lgtm!

code review and tested sharing files to multiple spaces with image embeddable

[nickpeihl]

nice tests! 💯

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)

[cqliu1]

Sorry for the pings everyone. I rebased this PR and am targeting main as a full feature independent of the shareable dashboard work. Instead of merging this as is, I'm waiting until #169870 is done, so we can enable sharing on the files management page and follow up with additional functional tests.