Allow lazy loading thumbnail

Problem/Motivation

Specially to defer offscreen images, we would like to allow lazy loading of downloaded thumbnail image.

Steps to reproduce

N/A

Proposed resolution

- Provide "Lazy load image" checkbox as third party settings for Thumbnail (video_embed_field_thumbnail) formatter.
- While rendering thumbnail we add 'loading' attribute to the thumbnail image

Remaining tasks

Providing the patch, we need reviews moving forward.

User interface changes

API changes

N/A

Data model changes

N/A