Add serialization/deserialization for rules

[khurram-ghani]

Describe the feature you'd like
Rules cannot currently be pickled. This will be useful, especially for BatchTrustRegion rules.

Is your feature request related to a problem? Please describe.
In order to continue an optimization from a previous run with BatchTrustRegion rules, one method to is to re-use a previous instance of the rule. Hence, the ability to serialize/deserialize the rule will be useful here.

Describe alternatives you've considered
The alternative method to continue an optimization is to create a new instance of the rule with the subspaces from a previous state.

Additional context
Related to #791.