Add `rollback_window` to elastic agent configuration with a default value of 7d

[pchila]

Add a new configuration key agent.upgrade.rollback_window of type time.Duration in elastic-agent configuration.
This key must be a duration string parseable by the time.ParseDuration function.

The default value of this setting is 7 days (or 168h since time.ParseDuration can parse up to hours as unit of time)

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[ycombinator]

By way of future-proofing, I suggest we go with agent.upgrade.rollback.window as the configuration setting name/path (notice the rollback.window instead of rollback_window in the last part of the path). This would let us add more rollback-related configuration settings in the future, if needed. WDYT @pchila?

[blakerouse]

I agree with @ycombinator. Other settings might come about in the future.

[pchila]

Reopened issue as #8767 will set the default to 0 temporarily.
Once #6885 is implemented the default value can be set again to 7d