[camerax] Implements `setFocusMode`

[camsim99]

Implements setFocusMode (also adds support for disabling auto-cancel to FocusMeteringAction host API implementation to accomplish this) + some minor documentation improvements based on discoveries I made while working on this :)

Fixes https://github.com/flutter/flutter/issues/120467.

~To be landed after: https://github.com/flutter/packages/pull/6110~ Done :)

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[camsim99]

@stuartmorgan This PR will bring camera_android_camerax to feature parity with camera_android. Should we bump the version to 1.0.0 here?

[stuartmorgan-g]

We aren't at 1.0 for any camera packages, so I would suggest leaving it at 0.x.