Add config.Provider.GetSkippedResourceNames

[ulucinar]

Description of your changes

This PR proposes to add a config.Provider.GetSkippedResourceNames method that returns a list of Terraform resource names that are available in the Terraform provider schema but that are either skipped or not included so that they are not generated as managed resources.

I've also considered:

• Directly generating the diff file when a path is configured (via provider configuration). But generating this diff, in my opinion, would be a supporting function (not among the core code generation pipelines), so instead of configuring a file path to store the diff and having Upjet generate it, I preferred a change to make Upjet be able to directly supply the required diff information
• Another approach would be not to touch Upjet but instead do the diff calculation in the providers themselves. In the provider, we already have the JSON schema from which we can extract the set of Terraform resources available and also the names of the Terraform resources for which we have generated the corresponding MRs (in the generated files under apis). But I found this approach too indirect and requiring further processing for a directly available piece of information. Thus I went with the small change proposed in this PR.

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Tested against upbound/provider-aws.

[ulucinar]

Closing/reopening the PR in an attempt to retrigger the CLA check...