Enable filtering of courses by `findable` and `running`

[paulrobertlloyd]

What did you do? (steps to reproduce)

I am trying to get a list of running and findable courses to show in the Find prototype.

What happened? (actual results)

There’s currently no filter options available for getting back a list of running and findable courses.

What should have happened? (expected results)

Include ability to filter by running and findable:

https://api.publish-teacher-training-courses.service.gov.uk/api/public/v1/recruitment_cycles/2021/courses/?page=1&per_page=20&filter[findable]=true&filter[running]=true

[defong]

Hey there, @paulrobertlloyd

@TobyRet has done half of the job
#1726

The other half I'm going to leave to @gpeng discretions.

[defong]

stale