Add support to opt-in to including fields in model access

[wattsyart]

Hello,

This is a draft PR to propose adding TomlModelOptions.IncludeFields property that supports fields in reflection model serialization (it is irrelevant for dictionary accessors, and ignored in those cases).

I need this for my game engine which uses fields over properties in performance critical areas, where the inlined backing fields add overhead.

[xoofx]

Thanks!