Rework BDX TransferSession to not encode PayloadHeader

[holbrookt]

Problem

Currently the bdx::TransferSession class outputs messages that already have a PayloadHeader attached at the beginning. However, this is incompatible with ExchangeContext::SendMessage(), which expects to receive a message without a PayloadHeader.

See:

connectedhomeip/src/protocols/bdx/BdxTransferSession.cpp

Line 46 in f18acae

CHIP_ERROR AttachHeader(chip::Protocols::Id protocolId, uint8_t msgType, ::chip::System::PacketBufferHandle & msgBuf)

connectedhomeip/src/messaging/ExchangeMessageDispatch.cpp

Line 49 in f18acae

PayloadHeader payloadHeader;

Proposed Solution

• TransferSession should not attach a PayloadHeader to the message
• TransferSession should have some way for the caller to get the MessageType of the message