[Config] Support `config` and `subset` fields for Extensions

[TristonianJones]

As part of the serializable configuration effort, all Extension
libraries should support config as a map field to indicate when
configuration data other than versions should be applied.

Likewise, Extension libraries should also support the subset
field akin to the stdlib field in order to make sure extensions
are trivial to subset when the implementor decides this is a worthwhile
feature to support.