Make AutoMerge name-spaced class instantiation in AutoMergeService searchable

What does this MR do and why?

Remove meta-programming from AutoMergeService. To make this easier to trace for anyone not intimately familiar with AutoMergeService. Issue: #492542 (closed)

This will also de-couple the class name from the strategy name so that we can properly namespace the merge train code giving it a bounded context.