DRM: Support Manifest Delivered DRM Keys

[itsjamie]

Now that #2735 is merged, the Fragment class encrypted property now reports true for DRM signalled keys.

During the implementation of this, something I didn't address was MultiDRM streams. Currently, the data model assumes one levelkey per fragment. However, it's possible for there to be multiple levelkey(s) per fragment.

MultiDRM manifests I have found are relatively common and currently are unsupported due to the fact that we only store a single level key on a fragment. This will need to change if we want to support manifest delivered keys in a multi-key system manifest.

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After we update the data model from an AES-128 Identity key centric style to support a DRM case, I see the steps roughly as;

• Discuss the extension property naming for the hls.js public config to signal a preference for manifest delivered keys or initialization segment delivered
• Add preemptive MediaKeySession negotiation for manifest delivered DRM keys if the preference is given to use them over EME "encrypted" event keys

Originally posted by @itsjamie in #2700 (comment)