Support for security scheme/auth

https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.3.md#security-scheme-object

Let's do a proposal first, as there are a few ways this could be implemented.

For now, the workaround is to write your own middleware that injects the appropriate headers to every outgoing request.

Example: https://github.com/apple/swift-openapi-generator/tree/main/Examples/auth-client-middleware-example

Start by reading the OpenAPI 3.0.3 Security Scheme Object specification linked in the issue, then review the Swift auth-client-middleware-example. Compare the possible implementation approaches and produce a proposal that defines the intended security-scheme/auth support and its scope.

Written by the indexing model from the issue text.