Cherry pick #2454 to release-4.0

[breezewish]

What problem does this PR solve?

Cherry pick #2454 to release-4.0

What is changed and how it works?

TiDB Dashboard.

Check List

Tests

• Manual test

Release note

Support setting public path for the embedded Dashboard.

[jebter]

/run-all-tests

[breezewish]

/run-all-tests

[codecov-commenter]

Codecov Report

Merging #2457 into release-4.0 will increase coverage by 0.56%.
The diff coverage is 82.62%.

@@               Coverage Diff               @@
##           release-4.0    #2457      +/-   ##
===============================================
+ Coverage        76.42%   76.99%   +0.56%     
===============================================
  Files              206      205       -1     
  Lines            21958    21936      -22     
===============================================
+ Hits             16782    16889     +107     
+ Misses            3905     3752     -153     
- Partials          1271     1295      +24     

Impacted Files	Coverage Δ
client/metrics.go	100.00% <ø> (ø)
pkg/apiutil/serverapi/middleware.go	73.07% <ø> (+2.70%)	⬆️
server/schedule/operator_controller.go	81.00% <ø> (ø)
server/server.go	77.53% <0.00%> (-0.35%)	⬇️
server/util.go	50.00% <0.00%> (-1.20%)	⬇️
tests/cluster.go	86.74% <ø> (+0.65%)	⬆️
tests/pdctl/helper.go	100.00% <ø> (ø)
pkg/component/manager.go	81.01% <25.00%> (-6.83%)	⬇️
server/api/region.go	73.83% <40.00%> (-0.72%)	⬇️
tools/pd-ctl/pdctl/command/config_command.go	72.01% <60.00%> (-0.52%)	⬇️
... and 41 more

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[breezewish]

/run-all-tests

[jebter]

/merge

[sre-bot]

/run-all-tests