Add PATCH/HEAD/OPTIONS support; fix trait method visibility and enum query params

[LiJoeAllen]

Summary

Three improvements developed while building an OpenAPI-to-feignhttp code generator (feignhttp-generator) that emits #[feign] trait clients and exercises them with wire-level integration tests.

1. PATCH / HEAD / OPTIONS method support (c9d5d69)

• codegen/src/enu.rs: add Patch / Head / Options to the Method enum plus from_str / to_str mappings.
• codegen/src/lib.rs: expose #[patch], #[head], #[options] attribute macros.
• reqwest supports these verbs natively; the generic method-string passthrough in func.rs needed no changes.
• New example examples/method.rs.

2. Generated trait client methods were private (f5cd066)

The #[feign] macro on a trait generates a shadow struct plus an inherent impl TraitName { ... } containing the request methods. trait_method_to_item_fn hard-coded method visibility to Visibility::Inherited (private), so:

• Upstream's own tests passed only because test code lives in the same module as the macro expansion.
• Any cross-module or cross-crate consumer got E0624: method is private — which makes externally generated clients unusable.

Fix: pass the trait's visibility into fn_to_streams_for_trait -> trait_method_to_item_fn and use it for the emitted methods.

3. Enums could not be used as query parameters (2902696)

filter_struct classifies any non-primitive type as "struct", routing enum query args into ser::to_map. MapSerializer::serialize_unit_variant returns Serialize("not support unit_variant"), so #[query("granularity")] g: MyEnum always failed at runtime, even though ValueSerializer already handles unit variants.

Fix:

• Add ser::to_pairs (PairsSerializer): ordered key-value collection where structs emit one pair per field, bare scalars/unit variants emit a single empty-key pair, None emits nothing, and newtype variants emit (variant, inner).
• Emit to_pairs in both struct-query sites of func.rs; an empty key falls back to the Rust argument name. Behavior for real structs is unchanged.

Testing

• Upstream suite: all tests pass except pre-existing mockito socket-bind environment failures (os error 10013) unrelated to these changes; all examples compile.
• New external consumer crate (module layout, cross-crate): 11/11 runtime tests green against a stub HTTP server, covering URL template substitution, path/query/header/form/multipart serialization, JSON decode with renames, octet-stream bytes, HEAD/OPTIONS, enum queries, and typed error parsing via ErrorKind::Status.

[LiJoeAllen]

@dxx 😄 O(∩_∩)O 😄

[dxx]

@LiJoeAllen Thanks for your PR.

[dxx]

@LiJoeAllen Hi, I think HEAD and OPTIONS aren't useful for feignhttp because response headers can't be retrieved. Can we remove them?

[LiJoeAllen]

ok

[LiJoeAllen]

Sure! Since feignhttp can't retrieve response headers, HEAD/OPTIONS indeed aren't useful here. I've removed them in #15, keeping only PATCH. Thanks!