Support for const schemas

[zumm]

Description

According to this schemes like that is valid:

    schema:
        const: 
            result: "ERROR"
            error_code: "ERROR__PET_NOT_FOUND"

Would be nice to have it supported by openapi-ts.

[mrlubos]

Yep! I've recently added primitive constants in https://github.com/hey-api/openapi-ts/releases/tag/%40hey-api/openapi-ts%400.61.1, this would be the next iteration as well as arrays I think