feat(avatar): add styles for default (medium) size in ionic theme (#29538)

Issue number: internal

---------

## What is the current behavior?
Avatar does not have any styles in the ionic theme.

## What is the new behavior?
- Adds background, border and font styles for avatar
- Adds the styles for the medium size and defaults the size to medium
- Adds e2e test for avatar sizes

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

## Other information

[Preview](https://ionic-framework-git-rou-10735-ionic1.vercel.app/src/components/avatar/test/size?ionic:theme=ionic)

packages/angular/src/directives/proxies.ts

@@ -210,14 +210,14 @@ export declare interface IonApp extends Components.IonApp {}
 
 
 @ProxyCmp({
-  inputs: ['mode', 'theme']
+  inputs: ['mode', 'size', 'theme']
 })
 @Component({
   selector: 'ion-avatar',
   changeDetection: ChangeDetectionStrategy.OnPush,
   template: '<ng-content></ng-content>',
   // eslint-disable-next-line @angular-eslint/no-inputs-metadata-property
-  inputs: ['mode', 'theme'],
+  inputs: ['mode', 'size', 'theme'],
 })
 export class IonAvatar {
   protected el: HTMLElement;