Switch to discovery client for managed resources

[sergenyalcin]

Description of your changes

This PR switches to discovery client for getting managed resources

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.

[turkenh]

Crosslinking a provider-kubernetes PR that could be related here: crossplane-contrib/provider-kubernetes#116

I couldn't have a chance to dive deep yet but just wanted to share in case they are related.

[ulucinar]

Thanks @sergenyalcin, lgtm. The discovery client based implementation will also help us for discovering API resources of a given category category such as the managed resources. We need this functionality to implement certain migration scenarios involving provider families as their migration targets.

Let's iterate on this discovery-client based implementation so that we make use of the meta.RestMapper interface. The problem we are addressing here is correctly determining the GVR for the given GVK and RestMapper implementations have the RestMapping and RestMappings methods that we can use to convert the GVK to GVR instead of manually filtering the discovered API groups.