[Enhancement] added support for custom type & format mapping

[wing328]

Based on #4931 (credits: @ShoeBoom) but use + as the separator

=========================

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

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cc @OpenAPITools/generator-core-team