Add context to download events for QoE metrics

[matvp91]

Have you read the FAQ and checked for duplicate open issues?
Yes

Is your feature request related to a problem? Please describe.
Yes, we're using downloadcompleted and downloadfailed to report requests to Mux data. These then map on the throughput events required for mux to calculate some QoE metrics.

Describe the solution you'd like
The following fields shall be optional and are merely used to provide additional metadata.

• Add requestType (net.NetworkingEngine.RequestType) to shaka.extern.Request if applicable. In combination with contentType, this'll then map on request_type param.
• Add bitrate to shaka.extern.Request, in order to populate request_labeled_bitrate
• Add videoWidth and videoHeight, will only be filled in when we fetch a video segment.

Although I'm not too sure we should introduce this as an extra set of key / value pairs considering the net engine is rather generic. An idea that plays in my head is to provide an optional shaka.extern.RequestContext (new) instead.

Describe alternatives you've considered
N/A

Additional context
N/A

Are you planning to send a PR to add it?
Yes

[avelad]

@matvp91 do you have any update?