feature: add Select api into SupernodeLocator

[lowzj]

Signed-off-by: lowzj [email protected]

Ⅰ. Describe what this PR did

Providing a way for users to chooses a supernode based on the giving key.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #1345 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1345      +/-   ##
==========================================
- Coverage   51.35%   51.34%   -0.02%     
==========================================
  Files         130      130              
  Lines        8576     8578       +2     
==========================================
  Hits         4404     4404              
- Misses       3804     3806       +2     
  Partials      368      368              

Impacted Files	Coverage Δ
dfget/locator/locator.go	100.00% <ø> (ø)
dfget/locator/static_locator.go	68.49% <0.00%> (-1.93%)	⬇️

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[wangforthinker]

LGTM