Add migration_id token to migrate events

Issue information

From user: coderdan

Related to !483 (merged)

Contribution record

Fork management

Problem/Motivation

I have a workflow that needs to trigger an action only when a particular migration has finished importing. Currently, the tokens provided by the migration events in eca_migrate don't expose the migration's plugin id, so there is no way to know which migration is executing.

Proposed resolution

Add a 'migration_id' token to the 'migration' ECA event that returns the id of the executing migration.

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