feat(avatar): add styles for soft shape in ionic theme (#29552)

Issue number: internal

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## What is the current behavior?
Avatar does not have styles for the `"soft"` shape in the ionic theme.

## What is the new behavior?
- Adds the styles for the soft shape (border radius)
- The `xsmall` and `small` sizes have a different border radius than the
larger sizes
- Adds e2e tests for the soft shape

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

## Other information

[Preview](https://ionic-framework-git-rou-10739-ionic1.vercel.app/src/components/avatar/test/shape?ionic:theme=ionic)