Add migrate process plugin

This uses an interesting implementation pattern such that a token value is expected to be set and then being returned to the event afterwards. In other cases, we have introduced separate action plugins to set the return value back to the event. Doing it with the clean-up is clever and we may well change other implementations to use something similar.

What I'm wondering, though: do we have to make the token name configurable? We could instead hard-code the token name, which would then just be one less decision a user has to make.

Apart from that, I've left a few comments in the MR.

Only one code suggestion, I've added the changed code block to the comment in the MR.

Thank you so much, this is now merged.

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