Better Coverage of SqlPackage parameters via DacServices

**Is your feature request related to a problem? Please describe.**
There are a multitude of SqlPackage parameters that are not available publicly to c# via DacServices - namely MaxParallelism!

I believe I worked around the issue by invoking DacServices.SetAmbientSetting via reflection, however, that feels like a no-good hack!

**Describe the solution you'd like**
Can SetAmbientSetting be made public? Or can more fully expose the full feature set (especially MaxParallelism) to consumers?

Start by examining the DacServices API and its SetAmbientSetting entry point, with particular attention to how SqlPackage parameters such as MaxParallelism are represented. Determine which parameters should be public and what existing API or compatibility constraints apply; done means consumers can access the needed settings without reflection.

Written by the indexing model from the issue text.