WithOpenApi generates annotations for endpoints without MethodInfo

[captainsafia]

Closes #44970.

Our current OpenAPI-implementation does not generate OpenAPI specs from endpoints that are derived from a RequestDelegate. For most scenarios, this is a reasonable approach as developers might not want endpoints used for health-checks to show up in their OpenAPI annotations. In some cases, it's helpful to display OpenAPI specs even if the endpoint is sourced from a RequestDelegate.

This PR makes it so that invoking WithOpenApi on an endpoint will generate OpenAPI info even if it is source from a request delegate.

app.MapGet("/authorize", (HttpContext context) => ...).WithOpenApi();

Will generate an OpenAPI operation that:

• contains no parameters
• emits no response
• respects tags, summary, and annotation attributes

[ilmax]

Will this allow health check endpoints to be exposed in the Swagger document

[captainsafia]

Will this allow health check endpoints to be exposed in the Swagger document

Not quite. Ran into a hiccup with this that is more or less related to #44209. Need to go back to the drawing board on this one :/....