Repository Custom Properties: Add getters and setters

[chapurlatn]

Repositories Custom Properties is a Beta Github feature.

Note about custom properties:
It allows to define and manage properties at organization level. And eventually let people set repositories properties.
For example, it's used to appy ruleset on dynamic set of repositories based and repositories custom properties values.

The client's parts to manage properties have been contributed:
#2986
#3020

But custom properties are also provided by Github when getting a repository or in Github events.
For example, it allows to take into account a given repository properties to adapt its processing by a Github app.

It suggest to add the property on Repository model and on PushEventRepository model.

(pr is comming)

[chapurlatn]

PR Add custom properties on Repository and PushEventRepository
#3065