Create transcoding provider client

[victorges]

Initial implementation should include AWS MediaConvert as an implementation.

• Should be configurable with a URL just like Object Store drivers
  • Maybe: Separate config for input/output S3 buckets (final logic will be: external URL -> S3 -> MediaConvert -> S3 -> final object store)
• Should receive the same input and produce the same output as the Catalyst VOD API

  
  package transcoding

type Provider interface { Transcode(req UploadVODRequest, progress func(float64)) (TranscodeStatusCompletedMessage, error) }

 - Should create the transcoding job and wait for completion synchronously
 - On the first version, it is OK to only accept S3 (s3:// URLs) for input and output
   * Probably separate: handle moving input/output files to/from S3. This should use the drivers library, but likely should not be part of the client logic 
 - Create some unit tests for this new client (can mock MediaConvert)

[cyberj0g]

Sounds good! Not sure if it applies in this case, but let's keep in mind that we may be increasing attack surface by adding indirect dependencies to go-livepeer, when working on new go-tools features.

[victorges]

@cyberj0g that makes sense! I hadn't thought of that.

One thing I could do is just implement this as a package in the catalyst-api repository instead, which will be the only one using this at first anyway. Then if we ever want to use it in other projects we can move it here and deal with the extra indirect dep on go-livepeer (which might as well be the project wanting to use it).

Wdyt?