Implement `Debug` w/ detailed manifest for `Reader`

[ok-nick]

Changes in this pull request

• Add Store field to ManifestStore to open access to all Store methods and allow constructing a ManifestStoreReport on the fly
• Implement Debug for Reader, providing the user with detailed debug information about the manifest (effectively exposing ManifestStoreReport in the unstable API)

Notes

• The Reader already implements Display, providing user-friendly information about the manifest, whereas Debug provides detailed info via ManifestStoreReport.
• How can we expose ManifestStoreReport::dump_tree and ManifestStoreReport::cert_chain in the unstable API?
  • A method for retrieving certificates from the manifest, Manifest::certificates, could be useful

Checklist

• [x] This PR represents a single feature, fix, or change.
• [x] All applicable changes have been documented.
• [x] Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.

[ok-nick]

Adding Store to ManifestStore causes a breaking change since ManifestStore no longer implements UnwindSafe nor RefUnwindSafe. This is because Store contains a trait object, which cannot guarantee unwind safety unless the trait object requires the traits mentioned or we mask it with AssertUnwindSafe. See https://github.com/rust-lang/rust/issues/65717, https://github.com/rust-lang/rust/issues/40628, and https://github.com/rust-lang/rfcs/pull/3260.

[ok-nick]

For now we require TrustHandlerConfig to be UnwindSafe and RefUnwindSafe, but ideally TrustHandlerConfig would either be (1) a generic on Store or (2) each impl is gated behind a feature flag for each platform.