Add support for aeson's fieldLabelModifier and constructorTagModifier

[carymrobbins]

Support (internally) for an extensible StringModifier type class which can be used to represent functions for use with fieldLabelModifier and constructorTagModifier and the type level.

Exposes FieldLabelModifier and ConstructorTagModifier as new constructors for AesonOption along with some potentially useful combinators in Data.Override.Aeson.Options.StringModifier.

While users could in theory create their own StringModifier types since this model is extensible, the API for doing so currently exists in an Internal module until we've proven this out to be stable enough to be made part of the public API.