This patch implements most functionalities for Media Foundation ClearKey
CDM, except the decryption part which will be implemented in bug 1870722
because there are still some unknown crashes happening during EME
playback we need to address.
Having this clearkey CDM allows us to start testing EME playback for the
media engine. Currently we only have very limited test coverage for
that.
Differential Revision: https://phabricator.services.mozilla.com/D174991
