Support custom content types with prepareMessage

[nplasterer]

The RN SDK didn’t support custom content types in the conversation.prepareMessage method!
The support for custom content types seems to have introduced a SendOptions parameter to conversation.send in which we can specify a custom JS content type. Most integrators never use conversation.send directly, instead using conversation.prepareMessage and the sendPreparedMessage method (imported directly from the SDK import { sendPreparedMessage } from "@xmtp/react-native-sdk")

[alexrisch]

Fixed in #198