[Bug] x-validate module type error when YAML schema: should be an atom

[GPrimola]

Hi,

I'm trying to use custom validators for specific types that require fine validations that OpenAPI doesn't support.
I came across the option x-validate, but when I try to use it I get a type error.
I'm defining my schemas in a YAML file and I think this option was designed with the module-based schema definition only.

10:45:27.528 [error] ** (ArgumentError) you attempted to apply a function on "MyApp.Spex.Validations.Regex". Modules (the first argument of apply) must always be an atom
    (erts 15.1) :erlang.apply("MyApp.Spex.Validations.Regex", :cast, [%OpenApiSpex.Cast{value: "*", schema: %OpenApiSpex.Schema%{type: :string, description: "If the format is regex, this option must be set specifying the Ruby Regular Expression to be used to parse and compose the structured message.", title: "Regex", "x-validate": "MyApp.Spex.Validations.Regex"}

schema:

...
regex:
  title: Regex
  description: this must be set specifying the Ruby Regular Expression to be used to parse.
  type: string
  x-validate: MyApp.Spex.Validations.Regex
...

Issue found at: https://github.com/open-api-spex/open_api_spex/blob/v3.21.2/lib/open_api_spex/cast.ex#L122

[GPrimola]

Fixed in #647