Add a Direct Update Stage.
Minor refactoring to how stages are built in the pipeline.
Refactor stage builders.

Stage builders all now take a schema - the precise schema table can be
worked out at runtime when build is called, rather than being part of
the constructor.
Rename PipelineEndpoint to StageEndpoint.

Rename #fields to #columns for consistency with everything else.
MysqlFileSink does not need columns defined in constructor.
Separate sinks and transformation builders for Schema table loads.
WIP