Make message grouping cusomizable

[madsroskar]

Motivation Currently, we group messages by sender, and optionally by a time interval defined by the user. However, some users have requirements that go outside of these two cases, and our current setup doesn't allow for customizing the grouping/groupStyles without creating a custom MessageList component.

Proposed solution Allow users to pass a function, similar to our getGroupStyles function to make the decision on how to group messages themselves.

Acceptance Criteria

• Implementation of the feature
• Documentation on how to make use of this
• Automated testing the default implementation

[vanGalilea]

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