Shaka Packager Raw Key support

[cstranex]

See #21 - Adds support for using raw key encryption.

Retains backwards compatibility with the existing Widevine encryption by adding an encryption_mode directive to the encryption configuration.

Also adds drm_label and skip_encryption to inputs.

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[cstranex]

@googlebot I signed it!

[cstranex]

@joeyparrish would you mind reviewing when you have a chance? 😄

[cstranex]

ping

[cstranex]

@joeyparrish would you mind reviewing?

[joeyparrish]

I'm sorry for the delay. We have too few people on too many projects at the moment. I'm looking at it now.

[joeyparrish]

This is really well done. Thank you so much for your contribution!

[joeyparrish]

I don't understand this part. What do you mean by default key and key id? Are you saying the associated key and key id from this raw key entry is a default applied to all tracks without their own label? (AUDIO, SD, HD, etc?)

[cstranex]

Yes that's correct, similar wording to the shaka-packager documentation but I omitted the last sentence which maybe clarifies it better?

[joeyparrish]

Okay, now I think I understand. How about we review docs at some point on our side for clarity, but I don't put you through any more trouble on this PR. This seems fine for now.

Thank you again for your contribution!

[cstranex]

This is really well done. Thank you so much for your contribution!

Always a pleasure :)