Add setting for forwarder's handler name

[roblaszczak]

Motivation / Background

Forwarder's handler name is now hardcoded. Because of it it's not possible to run multiple forwarders with one router.

Checklist

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• I wrote tests for the changes.
• All tests are passing.
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• Code has no breaking changes.
• (If applicable) documentation on watermill.io is updated.
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[zhjdhx]

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