unsafe recovery: Add newly created empty regions output for 6.5

[v01dstar]

What problem does this PR solve?

Issue Number: Ref #6715

What is changed and how does it work?

This is a manual cherry-pick for release 6.5.

In DR auto sync scenario, it is possible that a empty range hole is left in the secondary data center 
when the primary data center is totally unavailable and the replication mode is "sync recover" (super 
rare). In this case, unsafe recovery + flashback is not able to recover the cluster. The fallback 
is BR + PiTR.  While, to help the people faster identify whether the fallback is needed, we need to add 
empty regions info in the output of unsafe recovery.

If there are empty regions created, use BR+PiTR. If there are no empty regions, use flashback.

Check List

Tests

• No code

Code changes

N/A

Side effects

N/A

Related changes

Master branch PR: #6869

Release note

None.

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[v01dstar]

/cc @disksing @Connor1996

[v01dstar]

@mayjiang0203

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.11% 🎉

Comparison is base (2892b46) 75.54% compared to head (a2588c6) 75.65%.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-6.5    #6877      +/-   ##
===============================================
+ Coverage        75.54%   75.65%   +0.11%     
===============================================
  Files              328      328              
  Lines            33168    33176       +8     
===============================================
+ Hits             25056    25100      +44     
+ Misses            5944     5924      -20     
+ Partials          2168     2152      -16     

Flag	Coverage Δ
unittests	75.65% <100.00%> (+0.11%)	⬆️

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

Files Changed	Coverage Δ
server/cluster/unsafe_recovery_controller.go	82.79% <100.00%> (+0.57%)	⬆️

... and 24 files with indirect coverage changes

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[JmPotato]

Will using strings.Builder here gain better performance?