Add egress control through allowlist to prevent source code exfiltration

[AbhishekBhaskar]

What are you trying to accomplish?

Add domain-based egress control to the proxy to help prevent source-code exfiltration during Dependabot update jobs. Even when an attacker abuses a legitimately-configured registry token (e.g. Artifactory/Nexus), traffic to unknown hosts like evil.com should be visible and, eventually, blockable — something the earlier Method-header approach could not address.

This PR lands the observe (log-only) foundation: it builds a per-job allowlist and logs any request to a host not on it, without changing traffic behavior by default.

Anything you want to highlight for special attention from reviewers?

• Two independent flags on a new egress_allowlist config block: observe (log non-allowlisted requests, still allow) and enforce (drop with 403, matching the existing blockMetadataAPIHosts response). Both default false ⇒ fail-open, so existing jobs and GHES are unaffected.
• Allowlist source is intentionally minimal for this cut: static GitHub/Dependabot infrastructure domains + static per-ecosystem registry/CDN defaults keyed by PACKAGE_MANAGER. Credential-derived, OIDC-derived, and backend-supplied domains are deliberately deferred.
• Boundary-safe matching: new helpers.HostMatchesDomain uses a leading-dot suffix check so one entry covers subdomains (npmjs.org → registry.npmjs.org) while lookalikes like evilnpmjs.org are rejected.
• Handler placement: registered in proxy.go right after request logging and before all credential-injecting handlers, so drops/logs happen before any auth is added. Works for HTTP and MITM'd HTTPS.
• enforce is wired but off by default; the intended rollout is observe first, then flip enforce per-ecosystem.

How will you know you've accomplished your goal?

• Unit: HostMatchesDomain boundary matrix (subdomains, case/trailing-dot normalization, lookalike negatives); handler flag matrix (fail-open, observe-logs-allows, enforce-403, observe+enforce), GitHub-infra always allowed, per-ecosystem defaults applied and cross-ecosystem defaults rejected.
• Integration (proxy_test.go): end-to-end through MITM — non-allowlisted host returns 403 under enforce and 200 + * egress not allowlisted <host> log under observe.
• Full suite green: go build ./... and go test ./... pass across all packages.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.