[tools/shoestring] fix: allow the user to retain their node key

[Wayonb]

problem: migrating from symbol-bootstrap creates a new node key.
this causes the node to lose their delegated harvesters.
solution: allow the node to retain its node key during bootstrap import and cert renewal.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.23%. Comparing base (01049eb) to head (3ffb0eb).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1186      +/-   ##
==========================================
- Coverage   98.25%   98.23%   -0.02%     
==========================================
  Files         162      158       -4     
  Lines        6630     6515     -115     
  Branches      143      143              
==========================================
- Hits         6514     6400     -114     
+ Misses        116      115       -1     

Flag	Coverage Δ
explorer-rest	?
tools-shoestring	96.89% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...shoestring/shoestring/commands/import_bootstrap.py	100.00% <100.00%> (ø)
...oestring/shoestring/commands/renew_certificates.py	100.00% <100.00%> (ø)
...ols/shoestring/shoestring/internal/NodeKeyUtils.py	100.00% <100.00%> (ø)
tools/shoestring/shoestring/internal/Preparer.py	100.00% <100.00%> (ø)
...ing/shoestring/internal/ShoestringConfiguration.py	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

[AnthonyLaw]

All look good 👍🏼
Added some comment

[Jaguar0625]

👍

[Jaguar0625]

👍

[Jaguar0625]

i think this should be staticmethod too?

[Jaguar0625]

👍