client/v3/maintenance.go: Add Downgrade support to client

[lilic]

I used this for testing the downgrade feature, requesting it via the client works correctly, but downgrade itself fails for me. I am not sure if we want to merge this now already due to that or wait until we adjust the downgrade itself?

I setup my etcd cluster and requested the dowgrade via this client, from 3.5.0 -> 3.4.16. But in the downgrade code we call this function:

etcd/server/etcdserver/cluster_util.go

Lines 479 to 480 in e2d67f2

	// cluster version only keeps major.minor, remove patch version
	ver = &semver.Version{Major: ver.Major, Minor: ver.Minor}

this discards the patch version I passed so my downgrade fails. I am not sure why we want to do this, as I find it useful to be able to downgrade to the latest 3.4.16 for example?

cc @serathius @ptabor @retroflexer @hexfusion

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

[codecov-commenter]

Codecov Report

Merging #13083 (d563c76) into main (af6f3dc) will decrease coverage by 15.27%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main   #13083       +/-   ##
===========================================
- Coverage   67.90%   52.63%   -15.28%     
===========================================
  Files         437      325      -112     
  Lines       33831    26496     -7335     
===========================================
- Hits        22973    13945     -9028     
- Misses       8895    10838     +1943     
+ Partials     1963     1713      -250     

Flag	Coverage Δ
all	52.63% <ø> (-15.28%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
client/pkg/v3/types/id.go	0.00% <0.00%> (-100.00%)	⬇️
client/pkg/v3/types/slice.go	0.00% <0.00%> (-100.00%)	⬇️
pkg/contention/contention.go	0.00% <0.00%> (-100.00%)	⬇️
server/mvcc/backend/hooks.go	0.00% <0.00%> (-100.00%)	⬇️
client/pkg/v3/fileutil/dir_unix.go	0.00% <0.00%> (-100.00%)	⬇️
client/pkg/v3/fileutil/sync_linux.go	0.00% <0.00%> (-100.00%)	⬇️
pkg/grpc_testing/stub_server.go	0.00% <0.00%> (-93.55%)	⬇️
...er/proxy/grpcproxy/adapter/watch_client_adapter.go	7.14% <0.00%> (-92.86%)	⬇️
client/pkg/v3/fileutil/read_dir.go	0.00% <0.00%> (-90.00%)	⬇️
server/etcdserver/api/v2store/ttl_key_heap.go	12.50% <0.00%> (-87.50%)	⬇️
... and 286 more

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[gyuho]

lgtm thanks!