cmd: move threshold check to CLI level

[KaloyanTanev]

A recent commit introduced a misbehavior when omitting the optional --threshold flag of create dkg and create cluster commands.
Because the threshold configuration is tested before the threshold variable is assigned to the default value ceil(2*n/3), the flag is not optional anymore.

This PR fixes this bug by moving the checks at the CLI level and by updating the corresponding tests accordingly.

It also adds an input validation check on the ThresholdSplit and ThresholdSplitInsecure functions to ensure they are called with a threshold parameter greater than 1.

category: bug
ticket: none

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 6 lines in your changes missing coverage. Please review.

Project coverage is 57.69%. Comparing base (a23fd53) to head (ee14d70).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
tbls/herumi.go	0.00%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3297      +/-   ##
==========================================
+ Coverage   57.60%   57.69%   +0.08%     
==========================================
  Files         211      211              
  Lines       30604    30618      +14     
==========================================
+ Hits        17629    17664      +35     
+ Misses      11070    11047      -23     
- Partials     1905     1907       +2     

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[sonarcloud]

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 SonarCloud