Handle User Metadata in workflow start requests

[gow]

What changed?

We are now handling the User Metadata attached in the start workflow requests.

Why?

Users need a way to set arbitrary information on workflows and events. This would be later used in the clients to build custom tooling (ex: UI showing additional information about a timer event etc)

How did you test it?

Added new functional tests

Potential risks

N/A

Documentation

N/A

Is hotfix candidate?

No