Automatically extend plugin config schema with enabled

[joshdover]

Follow up to #60998

We should automatically add the enabled config option to plugin config schemas if they are missing.

Depends on #59083

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[pgayvallet]

We should automatically add the enabled config option to plugin config schemas if they are missing.

The chosen implementation for #59083 do not allow consumers to check if a specific key is present in a given instance of ObjectType, meaning that 'adding a key if not present' is not possible atm.

We would need either direct access to the props property,an API to list the keys of the schema, or any other way to check the 'is key present' condition.

[mshustov]

Already unnecessary, considering #89584