feat: Add native Go 1.23 Iterator support

[merchantmoh-debug]

This PR introduces native Go 1.23 iterator support to the entire library via a new code generator.

The Problem

Pagination in go-github currently requires verbose boilerplate:

opt := &github.RepositoryListByUserOptions{ListOptions: github.ListOptions{PerPage: 10}}
for {
	repos, resp, err := client.Repositories.ListByUser(ctx, "user", opt)
	// handle err
	for _, repo := range repos { ... }
	if resp.NextPage == 0 { break }
	opt.Page = resp.NextPage
}

The Solution: With this PR, users can simply write:

for repo, err := range client.Repositories.ListByUserIter(ctx, "user", opt) {
	if err != nil { log.Fatal(err) }
	fmt.Println(repo.GetName())
}

Implementation Details

github/gen-iterators.go: A new AST-based tool (similar to gen-accessors) that scans all List* methods, identifies their pagination patterns (Page/Cursor), and generates a corresponding *Iter method returning iter.Seq2.
It handles methods using standard ListOptions embedding and explicit Page fields.
It copies the opts struct to avoid mutating the caller's options during iteration.

github/iterators.go: The generated file containing hundreds of type-safe iterators.
Metadata Handling: Updated tools/metadata to exclude the generated iterators from API operation mapping validation, as they wrap existing operations.

Testing & Benchmarks

github/iterators_benchmark_test.go confirms that the iterator abstraction introduces negligible overhead compared to manual looping.

github/iterators_test.go verifies single-page, multi-page, and error handling scenarios.

Tests explicitly verify that the user's opts struct is NOT mutated by the iterator.

Documentation

Added github/example_iterators_test.go to provide a clear example in the Godoc.
This modernizes the library to leverage the latest Go features, significantly improving Developer Experience (DX) by eliminating error-prone pagination boilerplate.