api: reorganize permission API

[kyrofa]

This PR continues the reorganization of #188 by extracting the permission API into its own module.

Note that this introduces no behavioral changes, it's just moving code around.

[codecov]

Codecov Report

Merging #191 into master will increase coverage by 0.23%.
The diff coverage is 59.09%.

@@            Coverage Diff             @@
##           master     #191      +/-   ##
==========================================
+ Coverage   54.92%   55.15%   +0.23%     
==========================================
  Files          15       16       +1     
  Lines         579      582       +3     
  Branches       52       52              
==========================================
+ Hits          318      321       +3     
  Misses        247      247              
  Partials       14       14              

Flag	Coverage Δ
#unittests	55.15% <59.09%> (+0.23%)	⬆️

Impacted Files	Coverage Δ
sros2/sros2/verb/create_permission.py	0.00% <0.00%> (ø)
sros2/sros2/api/_permission.py	61.53% <61.53%> (ø)
sros2/sros2/api/__init__.py	65.00% <66.66%> (+1.53%)	⬆️

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[mikaelarguedas]

lgtm