Show avatar on message notification

[Goooler]

Checklist

• [X] I have used the search function to see if someone else has already submitted the same feature request.
• [X] I will describe the problem with as much detail as possible.
• [X] This issue only contains a request for one single feature, not multiple (related) features.

App version

v6.602

Problem you are trying to solve

There are no avatars showing on message notifications as Gmail app did, can we add this support feature?

Suggested solution

Add setLargeIcon on notification builder: https://github.com/thunderbird/thunderbird-android/blob/29d5220995f7281ad0cffe4c776141361dacc35a/app/core/src/main/java/com/fsck/k9/notification/SingleMessageNotificationCreator.kt#L24-L42

Ref: https://developer.android.com/develop/ui/views/notifications/expanded#large-style

Screenshots / Drawings / Technical details

No response