Add Key mgmt library and create a top-level sparse-merkle-tree library

[jimthematrix]

• moved the sparse merkle tree implementation code to a top-level package sparse-merkle-tree
• started a new top-level package keys to demonstrate usage of existing encrypted keys capabilities for the Babyjubjub curve

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 45.74%. Comparing base (39bd365) to head (f857524).

Files	Patch %	Lines
zkp/golang/pkg/key-manager/key/key-manager.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   45.09%   45.74%   +0.64%     
==========================================
  Files          14       16       +2     
  Lines         530      540      +10     
==========================================
+ Hits          239      247       +8     
- Misses        250      252       +2     
  Partials       41       41              

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[Chengxuan]

@jimthematrix I think we should just replace the key-manager.go under pkg directory with this file for now along with its test.

[Chengxuan]

the key manager itself looks good to me, test coverage dropped so will need to be addressed. One solution is to combine the key-manager in internal package with the one in pkg package.

@jimthematrix feel free to address that in #25 if that makes things easier on your side.

[jimthematrix]

thanks for the review @Chengxuan. there's coverage in the integration-test/e2e.go that's not counted because we don't have all the steps to set up the e2e test. I'm hoping once we have implemented #29 the coverage will improve (like here: #30)