Provider/migrate to plugin framework

[maksym-nazarenko]

This PR is pretty huge, but it holds several crucial things:

• refactoring of the original provider to be compatible with mux server (mux server is used to combine several providers)
• rework of main.go to combine providers
• add .gitattributes file to collapse go.sum in diff viewer by default
• migrate Scheduler resource to plugin framework to show how it will look like

Relates to #132 (see notes regarding Protocol5 vs Protocolv6)

[maksym-nazarenko]

hey @ddelnano a lot of changes here, but now it is possible to move resources on-by-one seamlessly to Protocol6 (aka framework)
Some interesting features came with this framework, like validators for lists, interface approach for resource, etc.
Though, some resource implementations were not in the best form, espacially when you have name as Terraform resource (I also implemented my resources in that way, I must admit I have to re-visit those implementations 😬 )

Take a look, when you have time.

[ddelnano]

Thanks for investigating how to use this new framework and starting on the scheduler resource! I had a few questions, but overall lgtm.

As for your question here, we could probably drop support for terraform 0.12 to 1.0 soon but since you've done the v6 to v5 translation already let's stick with this. It would be nice if the terraform registry provider metrics had client version to gauge this, but they are pretty bare bones unfortunately.

[maksym-nazarenko]

@ddelnano I'm okay with supporting older version - just wanted to make this more visible, since on the official page 0.x versions are not even an option for the CLI: