chore: playground init support specifying k3s and k3d-proxy images.

[yipeng1030]

fix #488

• playground init support specifying k3s and k3d-proxy images by two flags --k3s-image and --k3d-proxy-image.
• For example, you can use kbcli playground init --k3d-proxy-image=registry.aliyuncs.com/apecloud/k3d-proxy:5.4.4 to specify the image of k3d-proxy.
• fix a bug of remaining test file after running ut in cmd/cluster.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 52.94118% with 8 lines in your changes missing coverage. Please review.

Project coverage is 42.58%. Comparing base (e6282c9) to head (32a9866).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/cmd/playground/init.go	33.33%	4 Missing ⚠️
pkg/cmd/cluster/operations.go	0.00%	2 Missing and 1 partial ⚠️
pkg/cloudprovider/k3d.go	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #500      +/-   ##
==========================================
- Coverage   42.59%   42.58%   -0.01%     
==========================================
  Files         172      172              
  Lines       23328    23337       +9     
==========================================
+ Hits         9937     9939       +2     
- Misses      12255    12261       +6     
- Partials     1136     1137       +1     

Flag	Coverage Δ
unittests	42.58% <52.94%> (-0.01%)	⬇️

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[shanshanying]

Better to encode infos (k3sImage string, k3dProxyImage string) into a struct.