Add Integrations CI

[foundrytom]

What

Add an integrations CI run that allows us to test Pull Requests in context (eg with OTIO linker and Test=CMake).

Why

Otherwise, don't have a canary until after release, which is too late.

[elliotcmorris]

For clarity. This is about adding an integrations check to this repo (mediacreation) testing things that use mediacreation, to check we're not releasing and breaking everything. Similar to the big integrations file in openassetio.