cmd: add link and unlink

[nexustar]

What problem does this PR solve?

cannot run installed component directly

What is changed and how it works?

add command tiup link and tiup unlink

• tiup link add a soft link to target component in ~/.tiup/bin
• tiup unlink remove it

Check List

Tests

• Manual test (add detailed scripts or steps below)

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

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

Release notes:

NONE

[ti-chi-bot]

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[codecov-commenter]

Codecov Report

Merging #1587 (f15d488) into master (5c91cf9) will increase coverage by 34.92%.
The diff coverage is 41.02%.

@@             Coverage Diff             @@
##           master    #1587       +/-   ##
===========================================
+ Coverage   15.78%   50.71%   +34.92%     
===========================================
  Files         149      292      +143     
  Lines       15023    26398    +11375     
===========================================
+ Hits         2372    13387    +11015     
+ Misses      12133    10996     -1137     
- Partials      518     2015     +1497     

Flag	Coverage Δ
cluster	43.85% <ø> (?)
dm	26.19% <ø> (?)
integrate	50.71% <41.02%> (+34.92%)	⬆️
playground	13.53% <ø> (?)
tiup	15.85% <41.02%> (+0.06%)	⬆️
unittest	∅ <ø> (∅)

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

Impacted Files	Coverage Δ
cmd/link.go	31.81% <31.81%> (ø)
cmd/unlink.go	46.66% <46.66%> (ø)
cmd/root.go	72.60% <100.00%> (+0.38%)	⬆️
components/cluster/command/test.go	58.66% <0.00%> (ø)
components/dm/command/edit_config.go	80.00% <0.00%> (ø)
components/cluster/command/import.go	19.10% <0.00%> (ø)
components/dm/command/template.go	41.17% <0.00%> (ø)
pkg/cluster/operation/check.go	48.58% <0.00%> (ø)
components/playground/instance/tiflash_config.go	100.00% <0.00%> (ø)
components/dm/spec/logic.go	78.19% <0.00%> (ø)
... and 236 more

---

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[AstroProfundis]

/hold

We'll continue this after v1.7.0 been released.