cmd: add API partial deposit flow

[KaloyanTanev]

1. charon deposit sign: sign partial deposit -> publish to the api
2. charon deposit fetch: fetch deposits from the api -> aggregate signatures -> save full deposit locally

category: feature
ticket: #3992

[codecov]

Codecov Report

❌ Patch coverage is 22.67081% with 249 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.50%. Comparing base (ffc9dcd) to head (e4685dc).
⚠️ Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
app/obolapi/deposit.go	0.00%	101 Missing ⚠️
cmd/depositsign.go	39.06%	60 Missing and 18 partials ⚠️
cmd/depositfetch.go	33.33%	34 Missing and 8 partials ⚠️
cmd/deposit.go	0.00%	21 Missing ⚠️
cmd/cmd.go	0.00%	4 Missing ⚠️
app/obolapi/api.go	0.00%	2 Missing ⚠️
cmd/exit_delete.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4032      +/-   ##
==========================================
+ Coverage   53.49%   53.50%   +0.01%     
==========================================
  Files         224      246      +22     
  Lines       37353    39665    +2312     
==========================================
+ Hits        19981    21224    +1243     
- Misses      15249    16176     +927     
- Partials     2123     2265     +142     

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[pinebit]

LGTM for the first pass.

[sonarqubecloud]

Quality Gate failed

Failed conditions
15.4% Duplication on New Code (required ≤ 15%)

See analysis details on SonarQube Cloud