⚠ rename Operator API to ClusterExtension API

[joelanford]

Description

This PR updates the Operator API to be renamed to ClusterExtension, and updates docs terminology, variable names, etc. to align with the more generic "extension" term. This helps in a variety of ways:

1. It signals that OLMv1 is about more than operators. (Yes, there's still the overall Operator Framework and Operator Lifecycle Manager branding to deal with)
2. It gets us out of the conundrum of having Operator APIs in both OLMv0 and OLMv1 in different groups, which causes UX problems with the kubectl <verb> operators.
3. The Cluster prefix paves the way to introducing a namespace-scoped equivalent Extension API.

Reviewer Checklist

• API Go Documentation
• Tests: Unit Tests (and E2E Tests, if appropriate)
• Comprehensive Commit Messages
• Links to related GitHub Issue(s)

[codecov]

Codecov Report

Attention: 22 lines in your changes are missing coverage. Please review.

Comparison is base (8d428a5) 83.60% compared to head (cd8b3b6) 83.60%.

Files	Patch %	Lines
...nternal/controllers/clusterextension_controller.go	76.62%	17 Missing and 1 partial ⚠️
api/v1alpha1/zz_generated.deepcopy.go	40.00%	3 Missing ⚠️
cmd/manager/main.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #568   +/-   ##
=======================================
  Coverage   83.60%   83.60%           
=======================================
  Files          20       20           
  Lines         811      811           
=======================================
  Hits          678      678           
  Misses         92       92           
  Partials       41       41           

Flag	Coverage Δ
e2e	63.50% <62.13%> (ø)
unit	78.72% <69.00%> (ø)

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[everettraven]

Looks good to me. Left a couple comments on more nitpicky things, so feel free to address them or ignore them - totally up to you.

[everettraven]

LGTM