add manifest backend for static JSON release manifests

[jaemk]

Closes #74.

• Add a manifest feature (no new dependencies) gating backends::manifest: update from a static manifest.json served by any plain file server (S3 static hosting, a CDN, GitHub Pages, nginx), with no forge-specific API.
• ManifestSource implements ReleaseSource (and AsyncReleaseSource under async); the manifest::Update::configure() facade mirrors the other backends' builders, with sync and AsyncUpdate variants.
• Manifest schema 1 is the only accepted version; any other schema fails with Error::InvalidResponse naming the version. Unknown fields are ignored for forward compatibility.
• Resolve relative asset urls against the manifest URL's directory; absolute urls pass through. Skip non-semver version entries with a debug log, same as the forge backends.
• Map asset digest values (sha256:<hex>) to ReleaseAsset::digest(); they plug into the existing release-digest verification (checksums feature).
• Add specs/ref-manifest-backend.md (the manifest format contract), examples/manifest.rs, and the manifest feature to all Makefile CI lanes.

Usage:

self_update::backends::manifest::Update::configure()
    .manifest_url("https://example.net/releases/manifest.json")
    .bin_name("app")
    .current_version(cargo_crate_version!())
    .build()?
    .update()?;

See specs/ref-manifest-backend.md for the manifest schema and examples/manifest.rs for a runnable example.