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.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.79%. Comparing base (c86c93c) to head (063a601).

❗ Current head 063a601 differs from pull request most recent head 3e5173b

Please upload reports for the commit 3e5173b to get more accurate results.

Files with missing lines	Patch %	Lines
api/etcdserverpb/raft_internal_stringer.go	75.00%	2 Missing ⚠️

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Additional details and impacted files

Files with missing lines	Coverage Δ
api/etcdserverpb/raft_internal_stringer.go	81.72% <75.00%> (ø)

... and 20 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #18645      +/-   ##
==========================================
+ Coverage   68.77%   68.79%   +0.01%     
==========================================
  Files         420      420              
  Lines       35535    35535              
==========================================
+ Hits        24438    24445       +7     
+ Misses       9668     9663       -5     
+ Partials     1429     1427       -2     

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[jmhbnz]

LGTM - Thanks @ivanvc

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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