[CON-3298] Inject promotional content inside Teaser

If this is your first x-dash pull request please familiarise yourself with the contribution guide before submitting. ✔️

Why you are seeing this PR

This PR is the first piece of the puzzle for integrating Clips on the ft-app homepage. If you'd like you can familiarise yourself with the spike. We are going to inject Promotional Content through a property. Promotional content in the case of clips will be cp-content-pipeline Clip component. To not over-complicate things and not create unnecessary dependencies between cp-content-pipeline and x-dash we've decided to take the approach of injection.
The approach was previously discussed and agreed by @kavanagh & @rowanbeentje

Discuss features first ✔️
Update the documentation ✔️
Must be tested in FT.com and Apps before merge ✔️ Promotional content feature won't be used on ft.com but was tested of FT-App
No hacks, experiments or temporary workarounds ✔️
Reviewers are empowered to say no
Reference other issues - N/A
Update affected stories and snapshots - N/A
Follow the code style ✔️
Decide on a version (major, minor, or patch)