adds a new subscription endpoint to save to cache

[s-amann]

What this PR does

Before this PR:
The endpoint did not exist

After this PR:
The endpoint exists, and requests to it will cache the payload

Jira: ARO-6310
Link to demo recording:

Special notes for your reviewer

Checklist

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Approvers are expected to review this list.

• PR: The PR description is expressive enough and will help future contributors
• Code: Write code that humans can understand and Keep it simple
• Refactor: You have left the code cleaner than you found it (Boy Scout Rule)
• Upgrade: Impact of this change on upgrade flows was considered and addressed if required
• Deployment: The deployment process was considered and addressed if required
• Testing: New code requires new unit tests.
• Documentation: Is the documentation updated? Either in the doc located in focus area, in the README or in the code itself.
• Customers: Is this change affecting customers? Is the release plan considered?

[mbarnes]

LGTM. I'm surprised we can't generate the subscription structs with the autorest tool, but I don't see anything in /api/common-types from which to do so. Doesn't need to block initial support but would be a nice-to-have.