[TSP] Don't send the entire TSP-signed message in the message content's signature field

The TSP SDK's sign_anycast() function returns a single byte array that contains the TSP signature, header, and message payload all in one big contiguous chunk.
This is quite inefficient; ideally, we do not want to send the entire message content in the org.robius.tsp_signature field, because it is already included in the Matrix room message event content itself, meaning that the message payload actually gets sent twice.
The other problem with this is that there is a size limit on the content field, so adding large custom subfields is a nonstarter, which will happen for large messages like long text, images, files, etc.

Instead, we should separate out the TSP signature & header from the message payload itself, and include the non-payload bytes only in the content's org.robius.tsp_signature field.

Blocked on this issue from the TSP SDK. See that issue for more info.

Start with the TSP SDK issue 218 and the integration entry point that calls sign_anycast(). Trace how its byte array is placed in the Matrix event content's org.robius.tsp_signature field, then confirm how the message payload is already represented in the room event. Done means the signature field contains only the TSP signature and header after the SDK supports separating them.