[BITV] Add title to Avatar component

[JuliaKirschenheuter]

Problem

To show a tooltip on a avatar we currently use v-tooltip. An example can be found in https://github.com/nextcloud/nextcloud-vue/blob/master/src/components/NcAvatar/NcAvatar.vue#L112. Another example is the files_sharing app: #35654. Example here: https://github.com/nextcloud/server/blob/master/apps/files_sharing/src/components/SharingEntry.vue#L29

v-tooltip is not accessible and we want to migrate to native tooltips (title attribute) instead. An overview can be found in nextcloud-libraries/nextcloud-vue#2503.

Solution

Add a title prop to the Avatar component that will render as title attribute on the HTML element.