Draft: Multi-step elastic migration DSL

What does this MR do and why?

Adds name to migration index. This will be useful for exporting metrics and make debugging easier.

Using the above as a use case, this MR adds a DSL for multi-step migrations.

TODO:

• documentation
• mapping task specs
• mapping task dumping schema
  • MigrationRecord should have its own schema/mapping file
• Elastic::Tasks should be auto loaded

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