Update message interface.

[alstojanovic]

Describe the background of your pull request

What does your pull request do? Does it solve a bug (which one?), add a feature?

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Governance

• [ ] Documentation is added
• [ ] Test cases are added or updated

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• I understand that any contributions or suggestions I made may make it into the actual code. I've read the License https://github.com/alkem-io/Coordination/blob/master/LICENSE.
• I understand that submitting the pull request requires agreeing to and signing the contributor license agreement.

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