feat: support composition revisions

[jcogilvie]

Description of your changes

This PR adds support for Crossplane's composition revision system and automatic/manual update policies in
crossplane-diff, ensuring diffs accurately reflect how Crossplane will render XRs based on their configured
update strategy.

Composition Revision Resolution

When diffing XRs, crossplane-diff now intelligently resolves which composition version to use based on the
XR's compositionUpdatePolicy and compositionRevisionRef fields:

• Automatic Update Policy: XRs with compositionUpdatePolicy: Automatic (the default) always use the latest
  CompositionRevision for their composition. This mirrors Crossplane's behavior where automatic XRs are
  continuously upgraded to the newest composition version.

• Manual Update Policy: XRs with compositionUpdatePolicy: Manual use the specific CompositionRevision
  referenced in their spec.crossplane.compositionRevisionRef field. This allows users to see diffs for XRs that
  are pinned to a specific composition version.

• Fallback Behavior: If composition revisions cannot be found (e.g., for newly created compositions),
  crossplane-diff falls back to using the Composition resource directly.

New CLI Flag: --include-manual

The comp command now includes an --include-manual flag that controls which XRs are included in the diff
output:

• Default behavior (--include-manual=false): Only shows diffs for XRs with Automatic update policy,
  since these are the XRs that will actually be affected by composition changes.

• With --include-manual: Includes XRs with Manual update policy in the diff output, allowing users to
  see the potential impact if they were to upgrade these manually-pinned XRs.

This filtering ensures users see relevant results by default, while still allowing inspection of
manually-managed XRs when needed.

Implementation Details

• Added CompositionRevisionClient to handle fetching and resolving CompositionRevision resources
• Enhanced CompositionClient.FindMatchingComposition() to use revision-based resolution via
  resolveCompositionFromRevisions()
• Added update policy filtering in CompDiffProcessor.filterXRsByUpdatePolicy()
• The XR schema validator now strips the compositionRevisionRef field before validation, as it's set by
  Crossplane controllers and may not be present in all XRD schemas

Fixes #71.

I have:

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