Add support for custom_properties

[pavlovic-ivan]

Requested by @tabathad . In this PR i've added support for custom_properties. Basically, any key-value pair can be added in that object, however terraform will fail if non-existing custom property is set. It supports oss_lifecycle with values from the ticket. I've configured the property from org settings so we can support it. Do note that i've renamed the property from oss-lifecycle to oss_lifecycle - snake case used throughout the yaml configuration.

Because of this implementation, i've added our custom GR-OSS json schema for validation, because we will use this property, others probably will not.

Roll out involves adding Custom Properties read&write scope to the Github App. Per org

[jgiannuzzi]

I like the idea of having a custom schema that extends the generic one. However I would keep it (and its documentation) out of this repo and into the specific config repo for the corresponding org.

[pavlovic-ivan]

@jgiannuzzi I don't like separating documentation from the code. So i suggest documenting custom properties from a high-level, implementation level, here, and move the custom schema to our gr-oss repos. Sounds good? Is that a good compromise?

[jgiannuzzi]

Yes obviously they should be documented here, maybe even along a template of how to customise the schema. It's just the GR-OSS specific fields that I think don't belong to this repo (their schema and their docs)

[pavlovic-ivan]

@jgiannuzzi comments addressed

[jgiannuzzi]

I still think having some information on how to customise the schema would be useful, but that can be in a follow-up PR