Support struct field type remapping via --with-type

[tannergooding]

Extends --with-type (-wt) to accept a field-qualified key (Struct.field=Type) so the emitted type of an individual struct field can be overridden.

#559 originally proposed --remap-type Something.type=SomethingType, but --remap-type/--remap-field shipped in #736 with a different meaning (name disambiguation). Instead this reuses --with-type, whose semantics are already "use this type for the given declaration," so int Something.type; can be emitted as SomethingType Something.type;.

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Implementation reuses the existing WithType helper, which already performs the override and preserves the original Clang type as a [NativeTypeName] attribute — it was previously wired only for enum backing types. Field type emission already flows through the same GetRemappedTypeName(fieldDecl, …) path, so the hook is a FieldDecl sibling branch alongside the existing EnumDecl one.

• WithType gains an optional matchStar = true parameter; the enum call site is byte-for-byte unchanged.
• The * catch-all is intentionally not honored for fields — rewriting every struct field to a single type is meaningless, so the qualified Struct.field key must be explicit.
• Behavior is full-type replacement (the value is the complete desired C# type); the original type is preserved as [NativeTypeName] unless equivalent; unrelated fields are unaffected. This composes with the existing macro/enum-backing/struct-marker uses of --with-type, which use distinct qualified names.

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Added a focused root-level regression fixture (WithTypeFieldTest) using ValidateGeneratedCSharpLatestWindowsBindingsAsync, rather than adding to the 16-way matrix, to avoid golden churn. It asserts int type → public SomethingType type; with [NativeTypeName("int")] and that a sibling int other; field is untouched. No existing goldens change.

Validation: dotnet build -c Release produces 0 warnings; dotnet test -c Release is green (3746 generator + 56 interop/3 skipped + 11 CLI-parser tests).

Fixes #559