[WIP] Adds support of deployments into system jobs

[dubadub]

Modifies generic scheduler to accommodate System Deployments.

Things left to do:

• Get rid of code duplication: right now system scheduler copies all the methods from the generic one. Maybe there is a way to reuse methods.
• Fix failing tests: system scheduler tests weren't updated, pending on proper new system scheduler factory implementation.

Sharing early as I hope to get feedback on this PR from the maintainers.

[hashicorp-cla]

All committers have signed the CLA.

[dubadub]

Related to #3644

[tgross]

Hi @dubadub! Just wanted to let you know we're not ignoring you here... this is a big issue and something we had some in-flight work on that's getting delayed. So we want to make sure we give you actionable feedback. We'll try to come back to you here soon. Thanks for your patience!