[epic] Add Extension API

[ncdc]

The Extension API should be the API users use to instruct the cluster to install/upgrade/delete an extension/operator. This eventually replaces ClusterExtension to become the only user-facing API for extensions/operators.

This will be a namespace-scoped API.

• Add Extension API #596
• Make Extension reconciler use Extension.spec.serviceAccount #595
• Delete ClusterExtension API #592

Follow-ups from #596 and #598

• Follow-up(add extension API): spec.paused #630
• Follow-up(add extension API): Cleanup unnecessary kubebuilder-generated stuff #631
• Follow-up(add extension API): Remove reconcile-time validations and tests #632
• Follow-up(add extension API): Remove unused fields from Extension reconciler #633
• Follow-up(add extension API): Remove ExtensionInterface and related functions/implementations #634
• Follow-up(add extension API): Mirror paused/managed spec to the status #635
• Follow-up(add extension API): Use union discriminator pattern for spec.source #636
• Follow-up(add extension API): Refactor tests to be more table-driven #637
• Follow-up(add extension API): update sample #638

Follow-ups that likely need refactoring based on Helm direction:

• (Follow up): Integrate Kapp: Add e2e tests for extension reconciler #653
• (Follow up): Integrate Kapp: Improve resolution logic in extension reconciler #654
• (Follow up): Integrate Kapp: Improve status reporting in Extension #655
• (Follow up): Integrate Kapp: Add support for registryV1 bundles #656