feat: added resolve feature

[jeroenvervaeke]

Add resolve helpers for ReferenceOr

What's this?

Makes it easier to get the value of a reference. Whether you have an inline value or a $ref pointing to #/components/, just call .resolve(&openapi) and get back a &T.

Example

let resp = op.responses.responses.get(&StatusCode::Code(200)).unwrap();
let resolved_resp = resp.resolve(&openapi).unwrap();

let mt = resolved_resp.content.get("application/json").unwrap();
let schema = mt.schema.resolve(&openapi).unwrap();
// schema is the fully resolved Schema, whether it was inline or a $ref

Works with ReferenceOr<T>, ReferenceOr<Box<T>>, and Option<ReferenceOr<T>>.

Also added some iteration helpers like OpenAPI::operations() and PathItem::iter() while I was at it.

No breaking changes, everything's backward compatible.

[spikespaz]

I oppose this addition because I don't think it belongs in this library. Resolving references comprehensively is non-trivial, because they may be to arbitrary paths or other documents completely.

This belongs in downstream code, where the implementation only needs to satisfy the consumer's requirements specifically.