feat: make drm sessionType configurable in advanced DRM config

[valotvince]

Description

Following #2750 , I've stripped everything talking about playready recommendation and kept only the sessionType override

Added a new field in the DRM AdvancedConfig sessionType allowing to force a specific MediaKey session to be created with that sessionType. Particularly useful when we issue purchase licenses, and the whole decryption chain checks for the configuration to be nice & pretty (unlike com.microsoft.playready).

Screenshots (optional)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update

Checklist:

• I have signed the Google CLA https://cla.developers.google.com
• My code follows the style guidelines of this project
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have verified my change on multiple browsers on different platforms
• I have run ./build/all.py and the build passes
• I have run ./build/test.py and all tests pass

[shaka-bot]

All tests passed!