[SE-48] Support native messaging transfers

[dremin]

Summary

• Prevented conversation-transfer feature from being enabled when native messaging transfers are enabled
• Updated custom-transfer-directory feature to use the native interfaces for starting a messaging transfer when the native feature is enabled
• Updated conversation-transfer doc to list differences between it and the native feature

Checklist

• Tested changes end to end
• Updated documentation
• Requested one or more reviewers

[github-actions]

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