add "allow_alphas" config

[JarbasAl]

allow to install alpha versions of skills from the appstore

"skills": {
    "installer": {
      "allow_pip": true,
      "allow_alphas": true,
      "break_system_packages": false
    }
}

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 66.66%. Comparing base (c694bd9) to head (c2dfe04).
Report is 1 commits behind head on dev.

Files	Patch %	Lines
ovos_core/skill_installer.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #430      +/-   ##
==========================================
- Coverage   66.71%   66.66%   -0.06%     
==========================================
  Files          15       15              
  Lines        2584     2586       +2     
==========================================
  Hits         1724     1724              
- Misses        860      862       +2     

Flag	Coverage Δ
unittests	66.66% <0.00%> (-0.06%)	⬇️

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