api: fix the encoded path of region key (#2399)

[rleungx]

What problem does this PR solve?

cherry-pick #2399.

What is changed and how it works?

This PR uses UseEncodedPath to solve the %2F problem.

Check List

Tests

• Unit test

Release note

• Fix the 404 problem when using region key in pd-ctl

[codecov-commenter]

Codecov Report

Merging #2577 into release-3.0 will decrease coverage by 0.08%.
The diff coverage is 40.00%.

@@               Coverage Diff               @@
##           release-3.0    #2577      +/-   ##
===============================================
- Coverage        69.89%   69.81%   -0.09%     
===============================================
  Files              171      171              
  Lines            16983    16987       +4     
===============================================
- Hits             11870    11859      -11     
- Misses            4141     4150       +9     
- Partials           972      978       +6     

Impacted Files	Coverage Δ
server/api/region.go	68.33% <25.00%> (-0.74%)	⬇️
server/api/router.go	100.00% <100.00%> (ø)
pkg/tempurl/tempurl.go	57.14% <0.00%> (-14.29%)	⬇️
pkg/etcdutil/etcdutil.go	86.04% <0.00%> (-9.31%)	⬇️
server/schedulers/shuffle_hot_region.go	58.97% <0.00%> (-6.42%)	⬇️
server/region_syncer/client.go	78.94% <0.00%> (-2.64%)	⬇️
server/util.go	72.61% <0.00%> (-2.55%)	⬇️
server/leader.go	77.91% <0.00%> (-2.50%)	⬇️
server/cluster.go	81.72% <0.00%> (-1.02%)	⬇️
server/handler.go	51.87% <0.00%> (+0.50%)	⬆️
... and 2 more

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[disksing]

/merge

[ti-srebot]

/run-all-tests