add MemberDowngradeUpgrade failpoint

[siyuanfoundation]

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

#17118

The MemberDowngradeUpgrade failpoint first downgrades all members, and upgrades some members. This is to test no robustness issue during the whole downgrade-upgrade process, which can be stopped at any point.

Tested locally with

go test -run TestRobustnessExploratory -v --count 100 --failfast --timeout 5h

[k8s-ci-robot]

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.75%. Comparing base (fce823a) to head (c6e3a18).

Additional details and impacted files

see 23 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19125      +/-   ##
==========================================
- Coverage   68.79%   68.75%   -0.04%     
==========================================
  Files         420      420              
  Lines       35640    35640              
==========================================
- Hits        24518    24506      -12     
- Misses       9697     9707      +10     
- Partials     1425     1427       +2     

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