playground: auto tiproxy sign certs

[xhebox]

What problem does this PR solve?

User needs to manually set session-xxxxx for tidb, to make tiproxy migration works. However, we could just generate a self-signed cert automatically. There is no security implication: the only requirement for this cert, is that, it should be same on all tidb instances.

Check List

Tests

• Unit test
• Integration test
• [x] Manual test (add detailed scripts or steps below)

[2024/01/29 15:59:00.527 +08:00] [INFO] [session_token.go:198] ["signing cert is loaded successfully"] ["cert path"=/Users/xhe/.tiup/data/U2jjZcE/tiproxy.crt] ["key path"=/Users/xhe/.tiup/data/U2jjZcE/tiproxy.key]

• No code

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:

Enable tiproxy connection migration by default on playground

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[codecov-commenter]

Codecov Report

Attention: 34 lines in your changes are missing coverage. Please review.

Comparison is base (271d5ea) 50.19% compared to head (6059a2d) 55.30%.

:exclamation: Current head 6059a2d differs from pull request most recent head c0e1bee. Consider uploading reports for the commit c0e1bee to get more accurate results

Files	Patch %	Lines
components/playground/instance/tiproxy.go	0.00%	27 Missing :warning:
components/playground/instance/tidb_config.go	50.00%	3 Missing and 1 partial :warning:
components/playground/playground.go	25.00%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2372      +/-   ##
==========================================
+ Coverage   50.19%   55.30%   +5.11%     
==========================================
  Files         328      329       +1     
  Lines       35009    35059      +50     
==========================================
+ Hits        17571    19389    +1818     
+ Misses      15118    13334    -1784     
- Partials     2320     2336      +16     

Flag	Coverage Δ
playground	15.32% <22.73%> (-0.01%)	:arrow_down:
tiup	33.60% <ø> (ø)
unittest	22.24% <0.00%> (?)

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