feat: repository custom properties plugin

[Simon-Boyer]

Repository Custom Properties

Fixes #543.

This new plugin allows to define custom properties values using this API: https://docs.github.com/en/rest/repos/custom-properties.
The syntax is:

custom_properties:
- name: custom_prop1
  value: foo
- name: custom_prop2
  value: bar

[Simon-Boyer]

@decyjphr do you think you'll be able to look at this PR soon? I would actually need this for a project I am currently working on. Thanks a lot!