[FEATURE REQUEST] Allow to cache arbitrary metadata together with a lease

[lkubb]

Is your feature request related to a problem? Please describe.
Leases are typically associated with states that ensure an application has access to currently valid ones - e.g. a state/formula that manages a config file including MySQL credentials and restarts the application on changes. When a lease reaches its maximum TTL before the state is run again, the application tries to use invalid credentials.

One currently implemented solution to this is expiry events. If one has an engine module that periodically renews leases, the LeaseStore will catch that a lease can no longer be renewed and send an event, which can be reacted upon. Without having a mapping of all lease names to corresponding states, the only option is to run a highstate on the minion. If the expiry event contained information about the state that issued the lease, it would allow to selectively run only the necessary state(s).

Describe the solution you'd like
Introduce a meta field that is cached together with the lease and include it in expiry events. It should be able to contain arbitrary data.

Describe alternatives you've considered
Reacting to expiry events with a highstate.