SecurityScheme validation incorrectly rejects specifications using custom scheme value for authorization header

[rjelierse]

REQUIRED. The name of the HTTP Authorization scheme to be used in the Authorization header as defined in [RFC7235]. The values used SHOULD be registered in the IANA Authentication Scheme registry.

Source: https://spec.openapis.org/oas/v3.0.3#security-scheme-object

The specifications mention it SHOULD be a registered scheme, so, in my opinion, the library shouldn't fail validation of the specifications if it isn't one of the values currently checked for.

I'm happy to create a PR to mitigate this, if you think this is behaviour that needs correction.