etcdctl: add --from-key to watch command

[avanish-garg]

Fixes #22381

Problem

etcdctl get and etcdctl del both support --from-key (watch/get/delete all keys that
are byte-wise >= the given key), but etcdctl watch doesn't -- it can only watch an exact
key or a --prefix.

Change

• Added a --from-key flag to the watch command, mirroring the exact pattern already
  used by get/del (etcdctl/ctlv3/command/get_command.go, del_command.go).
• Added the same --prefix/--from-key mutual-exclusion check get/del already have.
• Updated the interactive-mode flag reset/reparse paths so --from-key behaves the same
  as the existing --prefix flag across repeated interactive watch invocations.
• Added an e2e test case in tests/e2e/ctl_v3_watch_test.go mirroring the existing
  --prefix test case, run against a real spawned etcd server with the built CLI.

The server already fully implements this for Watch requests: WithFromKey()'s sentinel
value (RangeEnd = []byte{0}) is already handled in
server/etcdserver/api/v3rpc/watch.go with a comment saying "support >= key queries",
and the client library (clientv3.WithFromKey()) already documents itself as applying to
'Get', 'Delete', 'Watch'. So this PR is purely additive CLI wiring -- no server or client
library changes needed.

Verified: go build ./..., go vet ./etcdctl/... clean, existing Test_parseWatchArgs
unit test still passes, new e2e test (TestCtlV3Watch) passes against a real spawned
server running the actual built etcdctl binary.

[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.

Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[avanish-garg]

cc @ptabor @fuweid — both of you have prior commits in watch_command.go (the file this PR touches), 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.