[Enhancement] added support for custom type & format mapping

Created by: ShoeBoom

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\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

Makes it possible to map format.

For example these args can be used to properly map the following:

"foo": {
    "enum": [
        "0",
        "1",
        "2"
    ],
    "type": "integer",
    "format": "byte"
}

generate -i openapi.json -o out -g kotlin --type-mappings integer_byte=kotlin.Byte

By default it would map foo to Int

this is similar to #2730 (closed)