Allow Fleet to be disable-able in 8.x

[JAndritsch]

Describe the feature:

A recent change made it so that all plugins must opt-in to being disable-able: #89584. Fleet is one of several plugins that do not currently support this behavior. Prior to this change, one could disable access to Fleet via xpack.fleet.enabled: false. That is now deprecated in 7.x and incompatible in 8.x.

An alternative way to "disable" plugins in Kibana is to simply delete them from the x-pack/plugins folder. That does not work for Fleet, however, as it seems other plugins depend on its source code. The changes referenced in the above issue make it so the Fleet UI and its functionality cannot be hidden globally for all users.

Is it possible to have Fleet opt-in to this behavior so that it can be disabled once again?

Describe a specific use case for the feature:

The environments where I deploy the Elastic Stack do not support the functionality offered by the Fleet plugin, so I disable it to prevent users from interacting with UI that does not work. Fleet is not the only plugin that I disable in 7.x.

The users and roles in these environments are not managed by me. I cannot control how user permissions are set in those environments and cannot use Kibana Feature privileges to globally disable the plugin for everyone in all environments.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)