Cocoapods integration

[nakajima]

Overview

Issue: xmtp/xmtp-react-native-old#10

This PR represents the changes needed to package xmtp-ios into an XMTP cocoapod.

The major changes are:

1. Replace references to 'xmtp/proto' with buf Use Buf to generate proto code #93
2. Use the XMTPRust cocoapod to replace gRPC and other non-cocoapod-compatible code such as some secp256k1 operations (xmtp-rust-swift)
3. Change ApiClient and other Swift classes to accommodate XMTPRust, specifically in KeyUtil.swift and for signature verification logic

Diagram

See https://github.com/xmtp/xmtp-rust-swift#diagram

Releases

From this branch we've released:

• 0.1.3-beta0 - this is the first XMTP cocoapod release, targeting XMTPRust 0.1.2-beta0 which did unnecessary JSON serialization in the Rust<>Swift boundary and had broken streaming (only got first item). Pagination info not returned by API.
• 0.2.0-alpha0 - do not use, this release did not properly handle Pagination info from query and therefore only returns the first 100 elements of a query
• 0.2.1-alpha0 - targets XMTPRust 0.2.2-beta0 - Does not use serialization in Rust<>Swift boundary, instead converts between RustVec and Data. Returns pagination info in query and has working streaming.

Tests

• Ensure all unit tests pass (with a local node)
• Added new PaginationTests which exercise Pagination, one of the trickier parts of the ApiClient migration to Rust code
• XCode cloud keeps failing, seems related to misconfigured XMTP scheme

[michaelx11]

@nakajima - thoughts on the Example app? I think it's half-configured for Cocoapods and half-configured for Swift Package Manager. I get redefinition errors unless I nuke all the pods in the Podfile.