[FS-507] Improve the Federated Welcome Message Request

[mdimjasevic]

An earlier PR #2368 introduced a federated welcome message request "mls-welcome" that beside a raw welcome message also included the list of recipients. This is not only redundant, but it also opens ground for abuse by allowing to include recipients that are not intended to receive the welcome message. This PR fixes the issue by updating the federated request to carry only the raw welcome message. The endpoint handler decodes the welcome message and also checks for key package references.

Tracked by https://wearezeta.atlassian.net/browse/FS-507.

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