Add message type to describe Event/Command/Other

[rj93]

Messages are frequently either considered to be Commands or Events, there is even a brief section in the docs about it.

However the message spec doesn't currently have anyway to differnaitate these types of messages.

This could potentially be useful in the code generators to either implement "server" or "client" implementations.

Servers would generate the listeners for commands, and emitters for events. While clients would generate the inverse.

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[ishyverma]

Hey! @rj93 can you tell me exactly how to give the message the type server or client ? I want to contribute to this issue. Thanks !!😊