Burn should allow BA to provide update feed source

[barnson]

• Describe the scenario and benefits that the feature supports.

Today, a bundle update feed is authored in a bundle and no callback allows the BA to override it.

• Describe how you're accomplishing the feature today (if possible).

Today, you'd have to handle downloading and parsing of a feed if you need a dynamic URI.

• Describe what you'd like the new feature to do.

Add IBootstrapperEngine.SetUpdateSource, analogous to IBootstrapperEngine.SetDownloadSource, that a BA can call during update detection.

[jchoover]

There is an existing request that I planned on working on in 4.x for changing OnDetectUpdateBegin to be a modifiable string for sczUpdateSource. Is that sufficient, or do we need the ability to assign the update source dynamically without any original authoring for an update source?