add proposal about better ImagePullJob usage

[ABNER-1]

Ⅰ. Describe what this PR does

Optimization solutions for imagePullJob in special scenarios:

1. Users may require that pre-download image only when P2P is active.
2. In P2P scenarios, it is not preferred to pre-download too many images globally at the same time.
3. Restrict the number of images being pre-downloaded simultaneously on a single node

Ⅱ. Does this pull request fix one issue?

about #1682 #1679 #1681

Ⅲ. Describe how to verify it

Ⅳ. Special notes for reviews

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.97%. Comparing base (0d0031a) to head (fbaed6b).
Report is 126 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1848      +/-   ##
==========================================
+ Coverage   47.91%   49.97%   +2.05%     
==========================================
  Files         162      192      +30     
  Lines       23491    24668    +1177     
==========================================
+ Hits        11256    12327    +1071     
- Misses      11014    11067      +53     
- Partials     1221     1274      +53     

Flag	Coverage Δ
unittests	49.97% <ø> (+2.05%)	⬆️

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