[Dashboards] Add Export flyout to dashboards

[nickpeihl]

WIP
Fixes #192913

Summary

Adds utility functions and a PoC of the "Export" functionality to show the "source" of a Dashboard. This "source" can be used to create or update dashboards using the HTTP endpoints.

Some notes:

• The current state of the dashboard is represented as JSON in the flyout, including any unsaved changes.
• It works with new, unsaved dashboards as well. So a user could add panels to a new dashboard and export the JSON without ever needing to save the dashboard.
• This feature is pending design updates.

export-poc.mp4

Checklist

Check the PR satisfies following conditions.

Reviewers should verify this PR satisfies this list as well.

• 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
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines

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[nickpeihl]

Superceded by #217904