feature: implement new avatar providers

[ryangjchandler]

This pull request adds support for custom implementations of AvatarProvider. The AvatarProvider is an object that is responsible for getting an avatar for a user.

By default, Filament will still use any uploaded avatars or avatars provided by getFilamentAvatar. If the avatar provided by these methods is null, it will fallback to the configured avatar provider.

Out of the box, Filament uses Gravatar as the default avatar provider but also provides one for ui-avatars.com (like Jetstream).

You can change the avatar provider used by modifying the filament.auth.avatar_provider configuration value.

Gravatar

'auth' => [
    'avatar_provider' => \Filament\AvatarProviders\GravatarProvider::class,
]

UI Avatars

'auth' => [
    'avatar_provider' => \Filament\AvatarProviders\UiAvatarProvider::class,
]