repository: set user agent for mirror requests

[AstroProfundis]

What problem does this PR solve?

Set user-agent for HTTP requests make by repository, the default is grab now.

Related changes

• Need to cherry-pick to the release branch

Release notes:

repository: set user agent for mirror requests

[codecov-commenter]

Codecov Report

Merging #1342 (620c882) into master (54ac4d5) will increase coverage by 28.85%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master    #1342       +/-   ##
===========================================
+ Coverage   25.88%   54.73%   +28.85%     
===========================================
  Files         264      290       +26     
  Lines       19713    21581     +1868     
===========================================
+ Hits         5103    11813     +6710     
+ Misses      13827     7918     -5909     
- Partials      783     1850     +1067     

Flag	Coverage Δ
cluster	43.33% <100.00%> (?)
dm	18.92% <100.00%> (?)
integrate	49.02% <100.00%> (+33.22%)	⬆️
playground	14.47% <100.00%> (?)
tiup	15.80% <100.00%> (+<0.01%)	⬆️
unittest	22.42% <0.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
pkg/repository/mirror.go	40.07% <100.00%> (+0.63%)	⬆️
pkg/cluster/operation/destroy.go	56.61% <0.00%> (ø)
components/playground/instance/tikv.go	67.34% <0.00%> (ø)
components/playground/instance/drainer.go	0.00% <0.00%> (ø)
components/playground/instance/instance.go	45.45% <0.00%> (ø)
components/playground/instance/tiflash_config.go	100.00% <0.00%> (ø)
pkg/cluster/operation/scale_in.go	58.27% <0.00%> (ø)
components/playground/instance/tiflash.go	59.09% <0.00%> (ø)
components/playground/instance/tidb.go	81.25% <0.00%> (ø)
pkg/cluster/operation/action.go	64.55% <0.00%> (ø)
... and 199 more

---

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[lucklove]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• lucklove

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[lucklove]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 58860d9