Feat/federated nodes

[AdamLBS]

This pull request introduces support for federated (cross-node) chat sessions in the app, along with UI improvements to distinguish federated chats and various code refactors to support these features. The main changes include updating session creation to handle federated addresses, enhancing the ChatView model to represent federated chats, updating the user list to prevent duplicate conversations, and improving the UI to clearly indicate external chats.

Federation support and session creation:

• The createSession function now accepts an optional recipientUserAddress for federated users, fetches device keys via a proxy endpoint when needed, and treats HTTP 202 as a successful session initiation. [1] [2] [3] [4]
• The session payload and device handling have been updated to support federated users, and the UserDevice model adds a fromFederatedJson constructor to handle federated device formats. [1] [2]

Model enhancements for federated chats:

• ChatView now includes a federatedAddress field, an isRemote getter, and logic to parse and serialize this information. [1] [2] [3] [4]
• PendingSession includes the otpkUsed field, and its parsing is updated to handle empty or missing values. [1] [2] [3]

User interface improvements for federated chats:

• The conversations list (ConversationsScreen) displays an "ext" badge and the federated address for remote chats, with distinct styling.
• The user list screen now prevents starting duplicate chats by excluding users already in conversation, and adds a tabbed interface for local and federated user search.

CocoaPods and iOS build integration:

• The iOS project adds a Podfile and updates .xcconfig files to integrate CocoaPods, preparing for dependencies required by federated features. [1] [2] [3]

Minor fixes and refactors:

• Minor code cleanups, improved comments, and UI tweaks (e.g., fixing border color usage and widget structure in the conversations screen). [1] [2] [3]

These changes collectively enable seamless support for federated messaging and improve the user experience when interacting with both local and remote users.