Draft: Add column merge_method to merge_trains table

What does this MR do and why?

Adds the column merge_method to the merge_trains table

Currently, merge trains only support normal merge commits. In order to support other merge methods, we need to keep track of which merge method each train car should use. This column will be used for a few things:

1. To keep track of how a train car's MR should be merged into the train reference.
2. To keep track of how a train car's MR should be merged into the target branch.
3. To ensure we don't mix incompatible merge methods (the default value, 0, will represent a "legacy method" that is incompatible with any other merge method)

See the proof-of-concept MR for more context.

Part of &4911 (closed)

How to set up and validate locally

1. Run the migration

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