--v2-deprecation flag: opt-in mode to validate that store-v2 has no user-content

[ptabor]

The PR adds --v2-deprecation flag that allows to opt-in for a stage of v2 compatiblity, as documented in:
https://github.com/etcd-io/etcd/issues/12913\

TODO: Will add tests to this PR, but asking for early feedback.

[codecov-commenter]

Codecov Report

Merging #12943 (bda0926) into master (e892d6d) will decrease coverage by 0.07%.
The diff coverage is 68.75%.

@@            Coverage Diff             @@
##           master   #12943      +/-   ##
==========================================
- Coverage   72.86%   72.78%   -0.08%     
==========================================
  Files         430      432       +2     
  Lines       34240    34286      +46     
==========================================
+ Hits        24948    24956       +8     
- Misses       7358     7375      +17     
- Partials     1934     1955      +21     

Flag	Coverage Δ
all	72.78% <68.75%> (-0.08%)	⬇️

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

Impacted Files	Coverage Δ
server/config/config.go	74.24% <ø> (ø)
server/etcdserver/api/membership/store.go	49.70% <ø> (ø)
server/embed/etcd.go	69.04% <33.33%> (-0.22%)	⬇️
server/etcdserver/server.go	79.41% <38.46%> (-0.41%)	⬇️
server/embed/config.go	70.26% <50.00%> (-0.27%)	⬇️
server/etcdserver/api/membership/storev2.go	66.66% <66.66%> (ø)
server/config/v2_deprecation.go	90.90% <90.90%> (ø)
server/etcdmain/config.go	83.66% <100.00%> (+0.60%)	⬆️
server/etcdserver/api/v3rpc/lease.go	71.83% <0.00%> (-8.46%)	⬇️
client/pkg/v3/tlsutil/tlsutil.go	83.33% <0.00%> (-8.34%)	⬇️
... and 24 more

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[hexfusion]

looks good thanks @ptabor

[ptabor]

Added e2e tests coverage in the last commit.