playground-ng: Support parallel download

[breezewish]

What problem does this PR solve?

close #xxx

What is changed and how it works?

• hardcode 4 parallel download by default

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• 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:

NONE

[ti-chi-bot]

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 63.80368% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 42.36%. Comparing base (7d025cb) to head (88bda36).

Files with missing lines	Patch %	Lines
components/playground-ng/boot_executor.go	62.32%	28 Missing and 24 partials ⚠️
components/playground-ng/main.go	71.43%	2 Missing and 2 partials ⚠️
components/playground-ng/playground.go	80.00%	1 Missing and 1 partial ⚠️
components/playground-ng/boot.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2676      +/-   ##
==========================================
+ Coverage   42.15%   42.36%   +0.21%     
==========================================
  Files         424      424              
  Lines       46888    47043     +155     
==========================================
+ Hits        19764    19927     +163     
+ Misses      24473    24428      -45     
- Partials     2651     2688      +37     

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