Make safe HTTP client options composable

[andrew]

Add per-host private-address allowlists to safehttp.Options and use the shared host-aware gate from fetch. Re-export WithSafeHTTP, WithHTTPClient, and WithTransport from the root package.

Closes #28
Closes #67

[Copilot]

Pull request overview

This PR makes “safe HTTP” configuration composable across the codebase by moving per-host private-network allowlisting into safehttp.Options, having fetch use the shared host-aware gate logic, and re-exporting client option helpers at the registries package root.

Changes:

• Add AllowPrivateHosts []string and CheckHostIP to safehttp to allow private-network IPs only for explicitly allowlisted hosts.
• Update fetch.WithAllowPrivateHosts to populate safehttp.Options and gate resolved IPs via safehttp.CheckHostIP.
• Re-export WithSafeHTTP, WithHTTPClient, and WithTransport from registries root and add coverage for these root options.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
safehttp/safehttp.go	Adds per-host private allowlist support, host-aware IP checks, and host normalization.
safehttp/safehttp_test.go	Adds tests covering allowlisted private hosts behavior and redirect refusal to unlisted private targets.
fetch/fetcher.go	Switches fetcher gating to safehttp.CheckHostIP and stores allowlist in safehttp.Options.
fetch/fetcher_test.go	Updates tests to reflect AllowPrivateHosts storage/composition behavior.
registries.go	Re-exports WithSafeHTTP, WithHTTPClient, and WithTransport at the package root.
registries_test.go	Adds regression tests ensuring root-level option re-exports behave as expected.

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