[Derived Fields] Add DerivedFieldMapper

[qreshi]

Is your feature request related to a problem? Please describe

To support schema on reads, a new mapper class will need to be added which represents derived fields in the mappings.

Describe the solution you'd like

Add a DerivedFieldMapper class which defines how the this type of field is index and searched. In our case, it will not be indexed and the search will deviate from traditional fields (details on this will be captured in a separate issue). The two main attributes we want in this mapper class is a type for the field and a script which defines how the field will be derived.

Related component

Search

Describe alternatives you've considered

No response

Additional context

No response