fix: misc missing properties and type fixes

[dmccartney]

This fixes a pile of bugs/missing fields (coming from partners exploring SDK integrations).

Here's the hit-list:

• Conversation is missing .createdAt and .context.metadata
• listBatchMessages is ignoring pageSize on iOS
• DecodedMessage is missing contentTypeId
• DecodedMessage is missing topic
  • ... from conversation.messages
  • ... from conversation.streamMessages
  • ... from listBatchMessages (future PR, requires xmtp-ios and xmtp-android fixes)
  • ... from streamAllMessages (future PR, requires xmtp-ios and xmtp-android fixes)

Along the way this also:

• adds a bunch of tests around adapting message and conversation types
• formalizes some of the JSON generation in the native code
• reduces the imprint of conversationId to merely part of the context (i.e. topic alone can identify the conversation)

[github-actions]

🎉 This PR is included in version 1.9.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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