Split production & mock plugins

[rebello95]

This PR splits the protoc-gen-connect-swift plugin into 2, both of which use the same set of options:

• protoc-gen-connect-swift: Generates interfaces and production implementations
• protoc-gen-connect-swift-mocks: Generates mock implementations

Both of these plugins depend on a shared ConnectPluginUtilities module, and the repo has been reorganized to be more concise at the top-level:

├── Connect-Swift-Mocks.podspec
├── Connect-Swift.podspec
├── Examples
│   ├── ElizaCocoaPodsApp
│   ├── ElizaSharedSources
│   ├── ElizaSwiftPackageApp
│   ├── README.md
│   ├── buf.gen.yaml
│   ├── buf.work.yaml
│   └── proto
├── LICENSE
├── Libraries
│   ├── Connect
│   └── ConnectMocks
├── Makefile
├── Package.resolved
├── Package.swift
├── Plugins
│   ├── ConnectMocksPlugin
│   ├── ConnectPluginUtilities
│   └── ConnectSwiftPlugin
├── README.md
└── Tests
    ├── ConnectLibraryTests
    └── ConnectPluginUtilitiesTests