Feature: Allow user to specify how field names are mapped in `Map()`

[leandrosansilva]

Similar to transformers, a new interface called FieldNameMapper is introduced, allowing custom ways to perform field name mapping during Map().

The rationale behind this PR is that the current mapping (change the case of the first field character) was not flexible for a project where the users are used to use the values included in the json tags, as they are more stable and part of more well tested APIs (all json based, but similar can be used for bson, yaml, etc.).