Turn oas3-unused-components-schema into a more versatile rule

[nulltoken]

User story.
OpenaApi 3.x Components section list a lot of reusable objects. But we currently only focus on schemas orphaned objects. We could do so much more!!!

Proposal:

• Slightly tweak the function to make it accept an array of reusable objects locations and process them all as potential sources. The given property may have to be tweaked as well ($?).
• Rename this rule into oas3-unused-components-object
• Extend the test suites to cover those additional paths

Would this proposal be considered as a possible addition:

• Create another issue targeting the oas2 ruleset (and another one for the soon to be released asyncapi ruleset)

[nulltoken]

@philsturgeon Thoughts?

[philsturgeon]

Yeah I think we could have:

• oas2-unused-definition
• oas3-unused-component
• async-unused-component

Let's do this as a followup task when #965 is done.

[P0lip]

I think we can consider this closed thanks to #1440