Add support for "instance" to "ggshield config get", "unset", and "list"

[agateau-gg]

This PR is the continuation of #670. #670 added support for ggshield config set instance <NAME>. This PR adds support for ggshield config get instance, ggshield config unset instance and make ggshield config list show the instance.

[codecov-commenter]

Codecov Report

Merging #678 (60ee5f3) into main (a2cff8d) will increase coverage by 0.02%.
The diff coverage is 96.00%.

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@@            Coverage Diff             @@
##             main     #678      +/-   ##
==========================================
+ Coverage   93.00%   93.02%   +0.02%     
==========================================
  Files         128      128              
  Lines        5663     5681      +18     
==========================================
+ Hits         5267     5285      +18     
  Misses        396      396              

Flag	Coverage Δ
unittests	93.02% <96.00%> (+0.02%)	⬆️

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Files Changed	Coverage Δ
ggshield/cmd/config/config_get.py	93.33% <85.71%> (+0.74%)	⬆️
ggshield/cmd/config/config_list.py	100.00% <100.00%> (ø)
ggshield/cmd/config/config_set.py	97.36% <100.00%> (+0.14%)	⬆️
ggshield/cmd/config/config_unset.py	100.00% <100.00%> (ø)