cmd: create dkg with testnet

[pinebit]

charon create dkg supporting testnet flags same as charon create cluster does.
Also, set default network to mainnet for charon create cluster for consistency with dkg.

category: feature
ticket: #4052

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.03%. Comparing base (fcb8c34) to head (0e1faec).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/createdkg.go	57.14%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4238      +/-   ##
==========================================
- Coverage   57.03%   57.03%   -0.01%     
==========================================
  Files         237      237              
  Lines       30857    30865       +8     
==========================================
+ Hits        17599    17603       +4     
- Misses      11021    11027       +6     
+ Partials     2237     2235       -2     

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