feat(fileserver): add Content-Digest header support (RFC 9530)

[VedantMadane]

Summary

Fixes #7901

This PR adds optional RFC 9530 Content-Digest header generation to the ile_server handler for static file responses.

Details

• Configurable via JSON content_digest array or Caddyfile content_digest / digest subdirectives.
• Supports standard sha-256 and sha-512 algorithms.
• Computes Content-Digest: sha-256=:...: / sha-512=:...: headers for static file responses.
• Includes unit tests in staticfiles_test.go.

[CLAassistant]

All committers have signed the CLA.

[steadytao]

Quick note. We need an AI assistance disclosure and CLA to be signed. although CLA assistant can be annoying sometimes, its fixed now :D

[steadytao]

Always nice to align with RFCs but we need to do it to-the-letter when doing so. This should also have far more tests; verify exact digest values and add integration coverage for GET, HEAD, 304, ranges, precompressed responses and read/seek failures. In particular, a 206 digest must cover the selected representation data rather than the complete source file.

[francislavoie]

I think this can cause the sendfile optimization to be disabled when used (but only for an HTTP server, no TLS because we don't have kTLS yet).

When you pass a raw *os.File to a *net.TCPConn using io.Copy, Go triggers a zero-copy fast path, using the sendfile(2) syscall.

Should be noted in the docs.

[VedantMadane]

Thank you @steadytao and @francislavoie for the detailed feedback!

I have updated the PR to address all your points:

1. RFC 9530 Range (206 Partial Content) Support: Updated calculateContentDigest to parse the single range Range: bytes=start-end header. On 206 Partial Content responses, Content-Digest is now computed over the selected representation range bytes rather than the full source file, fully complying with RFC 9530 §2.
2. Zero-Copy sendfile(2) Documentation: Added documentation notes in both struct doc comments and Caddyfile docs clarifying that enabling content_digest requires reading file bytes into memory to compute digests, which bypasses OS sendfile(2) zero-copy fast path on unencrypted HTTP connections.
3. Comprehensive Integration Test Suite:
   • Added TestContentDigestExactValues asserting exact base64 multihash strings (sha-256 & sha-512).
   • Added TestContentDigestIntegration test suite covering GET (200), HEAD (200), 206 Partial Content Range, 304 Not Modified, and Precompressed (gzip sidecar) requests.
4. CLA & AI Disclosure: CLA signed via CLA Assistant, and AI assistant pair-programming usage disclosed.