Reject unsigned transactions sent via RPC (bp #11282)

[mergify]

This is an automated backport of pull request #11282 done by Mergify

Cherry-pick of a7079e4 has failed:

On branch mergify/bp/v1.1/pr-11282
Your branch is ahead of 'origin/v1.1' by 1 commit.
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You are currently cherry-picking commit a7079e4dd.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
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	both modified:   core/src/rpc.rs

no changes added to commit (use "git add" and/or "git commit -a")

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[mergify]

automerge label removed due to a CI failure

[codecov]

Codecov Report

Merging #11286 into v1.1 will increase coverage by 0.0%.
The diff coverage is 95.8%.

@@          Coverage Diff           @@
##            v1.1   #11286   +/-   ##
======================================
  Coverage   81.1%    81.1%           
======================================
  Files        292      292           
  Lines      69066    69080   +14     
======================================
+ Hits       56077    56089   +12     
- Misses     12989    12991    +2