Allow TypeMapping (without transformation)

[mumenthalers]

The Docs States "No data transformation is ever performed.." which is totally fine.

Still it would be very useful to generate types with DateTime as Date as it is quite simple to add an angular http interceptor which would to the work.

something like

{
  "$schema": "node_modules/ng-openapi-gen/ng-openapi-gen-schema.json",
  "input": "my-api.yaml",
  "output": "src/app/api",
  "typeMapping": {
    "date-time": "Date"
  }
}

Without this feature we're forced to always do a manual Mapping and define a Mapped type like type MappedType = Omit<OriginalType, 'createdAt'> & {createdAt:Date} which is (1) cumbersome and (2) error prone (as nullability cannot be addressed and property renaming would lead to a runtime error).

I would be open to create a PR but only if you're actually willing to introduce this feature.