Currently we don't have a PipelineDefinition builder.

We have something similar in the fluent aggregate API, which mixes together the building of a PipelineDefinition with methods to execute the aggregate command.

Since there are places where an application needs to provide a pure PipelineDefinition (e.g. $facet or creating a read only view) it would be nice to have a standalone PipelineDefinition builder that was not tied to the fluent aggregate API.