v2.0: adds feature-gate code to enforce retransmitter signature verification (backport of #1840)

[mergify]

Problem

Need to verify if the shred is retransmitted by the expected parent node in Turbine tree.

Summary of Changes

Added feature-gate code to enforce retransmitter signature verification

---

This is an automatic backport of pull request #1840 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 79cb077 has failed:

On branch mergify/bp/v2.0/pr-1840
Your branch is up to date with 'origin/v2.0'.

You are currently cherry-picking commit 79cb077f10.
  (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:   turbine/src/sigverify_shreds.rs

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

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

[bw-solana]

LGTM