mirror: add tiup mirror show command to print current mirror address

[baurine]

What problem does this PR solve?

Currently, there has no explicit command to get the current mirror address.

(An non-explicit way: cat ~/.tiup/tiup.toml and it is non-documented)

What is changed and how it works?

Add an explicit command to show the current mirror address.

Check List

Tests

• Manual test (add detailed scripts or steps below)

> ./tiup mirror show
https://tiup-mirrors.pingcap.com/

> ./tiup mirror set /Volumes/T7/local-tiup-mirror 
Successfully set mirror to /Volumes/T7/local-tiup-mirror

> ./tiup mirror show
/Volumes/T7/local-tiup-mirror

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

- Add `tiup mirror show` command to display the current mirror address

[codecov-commenter]

Codecov Report

Merging #1317 (5de69f9) into master (1d16f0d) will increase coverage by 29.51%.
The diff coverage is 55.55%.

@@             Coverage Diff             @@
##           master    #1317       +/-   ##
===========================================
+ Coverage   26.25%   55.76%   +29.51%     
===========================================
  Files         263      289       +26     
  Lines       18951    20827     +1876     
===========================================
+ Hits         4975    11615     +6640     
+ Misses      13204     7379     -5825     
- Partials      772     1833     +1061     

Flag	Coverage Δ
cluster	42.26% <ø> (?)
dm	26.54% <ø> (?)
integrate	50.14% <55.55%> (+33.65%)	⬆️
playground	24.16% <ø> (?)
tiup	16.52% <55.55%> (+0.03%)	⬆️
unittest	22.77% <55.55%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
cmd/mirror.go	48.80% <55.55%> (+0.12%)	⬆️
components/playground/instance/tiflash_config.go	100.00% <0.00%> (ø)
components/playground/instance/tiflash.go	59.09% <0.00%> (ø)
pkg/cluster/operation/operation.go	78.26% <0.00%> (ø)
components/playground/instance/tidb.go	81.25% <0.00%> (ø)
pkg/logger/audit.go	53.33% <0.00%> (ø)
...onents/playground/instance/tiflash_proxy_config.go	81.81% <0.00%> (ø)
components/playground/instance/ticdc.go	0.00% <0.00%> (ø)
pkg/logger/logger.go	100.00% <0.00%> (ø)
components/playground/instance/tikv_config.go	100.00% <0.00%> (ø)
... and 208 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1d16f0d...5de69f9. Read the comment docs.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• AstroProfundis

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by writing /lgtm in a comment.
Reviewer can cancel approval by writing /lgtm cancel in a comment.

[AstroProfundis]

/merge

[ti-chi-bot]

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

Commit hash: 5de69f9