etcdserver: fill a response header in auth RPCs

[mitake]

Fix #7691

/cc @idonin @heyitsanthony

[xiang90]

probably we should add some tests around this.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6ebadda). Click here to learn what that means.
The diff coverage is 81.81%.

@@            Coverage Diff            @@
##             master    #7716   +/-   ##
=========================================
  Coverage          ?   75.35%           
=========================================
  Files             ?      330           
  Lines             ?    25968           
  Branches          ?        0           
=========================================
  Hits              ?    19567           
  Misses            ?     4984           
  Partials          ?     1417

Impacted Files	Coverage Δ
etcdserver/api/v3rpc/auth.go	71.05% <81.81%> (ø)

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[heyitsanthony]

this isn't guaranteed to give the revision at the time of the update/request, if that matters