client/v3: add ResumeMutex to concurrency package

[avanish-garg]

Fixes #22382

Problem

concurrency.Election has ResumeElection, for reconstructing an election handle after
a process restart (used together with Session's WithLease option for a surviving
lease). concurrency.Mutex has no equivalent -- a restarted process that was holding a
lock has no way to get a working *Mutex handle back for it; it can only start over with
a fresh Lock() call, losing the distinction between "I already own this lock" and "I
need to compete for it."

Change

• Added ResumeMutex(s *Session, pfx, myKey string, myRev int64) *Mutex, mirroring
  ResumeElection's exact shape and doc-comment style.
• Added TestResumeMutex to tests/integration/clientv3/concurrency/mutex_test.go,
  mirroring the existing TestResumeElection pattern: locks a mutex, reconstructs a
  second *Mutex handle purely from the session + observed key/revision (simulating a
  process restart), and verifies IsOwner() still recognizes it as the owner and
  Unlock() correctly deletes the real server-side key.

Mutex's myKey/myRev fields are structurally identical to Election's
leaderKey/leaderRev, and IsOwner()/Unlock() only depend on those two values being
set correctly -- they have no dependency on which constructor produced them, so this is a
small, mechanical port of a pattern already proven for Election.

Verified: go build ./client/v3/..., go vet ./client/v3/concurrency/... clean, full
existing concurrency integration test suite passes alongside the new test (11/11,
including TestResumeElection, TestMutexLockSessionExpired, TestMutexUnlock, and the
package's Example* tests).

[kubernetes-prow]

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[kubernetes-prow]

Hi @avanish-garg. Thanks for your PR.

I'm waiting for a etcd-io member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[avanish-garg]

cc @ptabor @fuweid — both of you have prior commits in concurrency/mutex.go and concurrency/election.go (this PR mirrors ResumeElection's existing pattern for Mutex), so tagging in case this area is useful for you to look at. Also, this PR is gated behind needs-ok-to-test — would appreciate an /ok-to-test if it looks reasonable.