feat(avatar): add disabled property (#30284)

Issue number: internal

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## What is the current behavior?
Avatar does not have a disabled state for the ionic theme.

## What is the new behavior?
- Added styles for ionic theme disabled state
- Added states e2e test & snapshots

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

## Other information

<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->

[Preview](https://ionic-framework-git-rou-11728-ionic1.vercel.app/src/components/avatar/test/states?ionic:theme=ionic)

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Co-authored-by: Brandy Smith <brandyscarney@users.noreply.github.com>
Co-authored-by: ionitron <hi@ionicframework.com>