api: address revive unexported-return issues

[ivanvc]

This is the first pull request aiming to enable the unexported-return revive lint rule from golangci-lint.

The aim is to ensure that the structs returned from exported functions are exported, too.

In this pull request, the following structs are now exported:

• etcdserverpb.TxnRequestStringer, previously txnRequestStringer, as it is returned by the exported function NewLoggableTxnRequest().
• etcdserverpb.LoggablePutRequest, previously loggablePutRequest, as it is returned by the exported function NewLoggablePutRequest().

Part of #18370.

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

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.77%. Comparing base (c1976a6) to head (9142964).

❗ Current head 9142964 differs from pull request most recent head 3654d03

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Additional details and impacted files

Files with missing lines	Coverage Δ
api/etcdserverpb/raft_internal_stringer.go	81.72% <100.00%> (ø)

... and 28 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #18645      +/-   ##
==========================================
- Coverage   68.78%   68.77%   -0.02%     
==========================================
  Files         420      420              
  Lines       35539    35539              
==========================================
- Hits        24446    24442       -4     
- Misses       9665     9668       +3     
- Partials     1428     1429       +1     

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[jmhbnz]

LGTM - Thanks @ivanvc

[ivanvc]

/retest

[ivanvc]

/retest

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahrtr, ivanvc, jmhbnz, serathius

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