3542098: Added image selection to the MastodonApiEntity forms.

Added the image_fields and image_style properties to the MastodonApiEntity schema and entity class. Updated the MastodonApiEntityForm to include the image_fields and image_style selection interface. Fixed a small issue in the access check class that meant if no bundle was selected then no entity push form was available. Updated the MastodonApiEntityPushForm to include the image field selection and image alt text. Updated the submit handler on the form to create the media entity whilst creating the status update.

Closes #3542098