feat: Custom security schemes

[jeanschmitt]

Summary

Addresses a FIXME (in parse_security.go) about a too strict authentication schemes enum validation.

This PR introduces a new configuration field, authentication_schemes, used to configure allowed authentication schemes.
By default, the ones defined in https://www.iana.org/assignments/http-authschemes/http-authschemes.xhtml are considered.

In addition, a security type is generated for custom securities, making it possible to access the Roles defined in the specification from custom security implementations (which is especially useful with non-standard authentication schemes).

Breaking change

This PR introduces a breaking change in the generated SecurityHandler signature for custom securities:

type SecurityHandler interface {
	HandleCustom(ctx context.Context, operationName OperationName, req *http.Request) (context.Context, error)
}

// becomes

type SecurityHandler interface {
	HandleCustom(ctx context.Context, operationName OperationName, t Custom) (context.Context, error)
}

type Custom struct {
	Request *http.Request
	Roles   []string
}

I think this change is worth it, because it has little impact (req => t.Request), and facilitates future extensions for custom securities.