Allow adding emoji reactions

[nukeop]

Is your feature request related to a problem? Please describe. I want my bot to be able to add emoji reactions

Describe the solution you'd like As above

Describe alternatives you've considered N/A

Additional context I couldn't find anything about it in the client docs.

[AndrewKvalheim]

In the interim here’s an example of how to do it manually.

[turt2live]

This is currently available through the unstableApis and just needs to be moved to MatrixClient now that it's stable in the spec.