adds shred::layout::get_signed_data (backport #29438)

[mergify]

This is an automatic backport of pull request #29438 done by Mergify.
Cherry-pick of 50afb80 has failed:

On branch mergify/bp/v1.14/pr-29438
Your branch is up to date with 'origin/v1.14'.

You are currently cherry-picking commit 50afb80f5.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   ledger/src/shred/legacy.rs
	modified:   ledger/src/shred/merkle.rs
	modified:   ledger/src/shred/shred_code.rs
	modified:   ledger/src/shred/shred_data.rs
	modified:   ledger/src/shred/traits.rs
	modified:   ledger/src/sigverify_shreds.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   ledger/src/shred.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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