broker: different roles for different signers

[Evanfeenstra]

in the HELLO message, we should include a "role" as the payload

An optional byte in the HELLO payload to say what "role" it is

• 0: receive only
• 1: can send also
• maybe we need more roles, like for onchain stuff vs lightning stuff?

Store the role in the Connections map

Sort VLS message types by "send" or "receive" and only send "send" msgs to the phone (or any with the "sender" role)