Merge EventPipePayloadDecoder.DecodePayload and EventProvider.DecodeObject

[brianrob]

There are two event decoder paths in EventSource. We should look to merge them.

[tommcdon]

@sywhang @jorive is this work happening as part of IPC event pipe? If no, any objections to moving this to future?

[sywhang]

This isn't part of the IPC event pipe as this issue is mostly (if not all) refactoring managed code whereas the IPC work is all native part. I have no objections on moving this to future.