Update DefaultCodeGen to allow additional primitive types

Created by: steco

If a string field is specified with a format which is also defined using --typeMappings, it will be treated as a primitive type

For example, if you run the following command:

java -jar openapi-generator-cli.jar generate [options] --type-mappings dateorcutlabel=CutLabel

The generated code will assume there is a class CutLabel and use it for any parameter which has a specific similiar to this:

{ "name": "effectiveAt", "in": "query", "description": "Optional. The effective date of the data", "required": false, "type": "string", "format": "dateorcutlabel" }

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)