Support agent available rollback fields

[pchila]

What is the problem this PR solves?

This PR handles new available_rollbacks field sent by elastic-agent during checkin (implemented in PR elastic/elastic-agent#11143).
This information will be consumed by the Fleet UI to allow a manual rollback of recently upgraded agents.

How does this PR solve the problem?

This PR introduces new available_rollbacks field both in CheckinRequest and pendingT structs as a slice of AvailableRollbacks and a JSON array of objects respectively.
On Elasticsearch the documents in .fleet-agents will have a new field upgrade.rollbacks similar to:

...
"upgrade": {
            "rollbacks": []
  },
..

How to test this PR locally

Refer to PR elastic/elastic-agent#11143 How to test this PR locally section.

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

• Relates Support manual rollback for Fleet-managed agents elastic-agent#11143

[blakerouse]

I am actually good with this PR. I don't want to override @michalpristas comments about the available rollbacks, but if he also agrees to keep them seperate then I am +1

[michalpristas]

I'm good with it paolo explained reasoning offline

[cmacknz]

Looks good besides my one remaining comment.

When we are done with these changes, it would be good to update the horde drones to report as having available rollbacks so that the scale tests exercise these code paths.