feat: add comprehensive Rust implementation with feature parity

This commit completes the Rust implementation of mail2couch with full feature
parity to the Go version, including:

- Complete IMAP client with TLS support and retry logic
- Advanced email parsing with MIME multipart support using mail-parser
- Email attachment extraction and CouchDB storage
- Sync mode implementation with deleted message handling
- Enhanced error handling and retry mechanisms
- Identical command-line interface with bash completion
- Test configurations for both implementations

The Rust implementation now provides:
- Memory safety and type safety guarantees
- Modern async/await patterns with tokio/async-std
- Comprehensive error handling with anyhow/thiserror
- Structured logging and progress reporting
- Performance optimizations and retry logic

Test configurations created:
- rust/config-test-rust.json - Rust implementation test config
- go/config-test-go.json - Go implementation test config
- test-config-comparison.md - Detailed comparison documentation
- test-both-implementations.sh - Automated testing script

Both implementations can now be tested side-by-side with identical
configurations to validate feature parity and performance.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

rust/config-test-rust.json

@@ -0,0 +1,74 @@
+{
+  "couchDb": {
+    "url": "http://localhost:5984",
+    "user": "admin",
+    "password": "password"
+  },
+  "mailSources": [
+    {
+      "name": "Rust Wildcard All Folders Test",
+      "enabled": true,
+      "protocol": "imap",
+      "host": "localhost",
+      "port": 3143,
+      "user": "testuser1",
+      "password": "password123",
+      "mode": "archive",
+      "folderFilter": {
+        "include": ["*"],
+        "exclude": ["Drafts", "Trash"]
+      },
+      "messageFilter": {
+        "subjectKeywords": ["meeting", "important"],
+        "senderKeywords": ["@company.com"]
+      }
+    },
+    {
+      "name": "Rust Work Pattern Test", 
+      "enabled": true,
+      "protocol": "imap",
+      "host": "localhost",
+      "port": 3143,
+      "user": "syncuser",
+      "password": "syncpass",
+      "mode": "sync",
+      "folderFilter": {
+        "include": ["Work*", "Important*", "INBOX"],
+        "exclude": ["*Temp*"]
+      },
+      "messageFilter": {
+        "recipientKeywords": ["support@", "team@"]
+      }
+    },
+    {
+      "name": "Rust Specific Folders Only",
+      "enabled": true,
+      "protocol": "imap",
+      "host": "localhost", 
+      "port": 3143,
+      "user": "archiveuser",
+      "password": "archivepass",
+      "mode": "archive",
+      "folderFilter": {
+        "include": ["INBOX", "Sent", "Personal"],
+        "exclude": []
+      },
+      "messageFilter": {}
+    },
+    {
+      "name": "Rust Subfolder Pattern Test",
+      "enabled": false,
+      "protocol": "imap",
+      "host": "localhost",
+      "port": 3143,
+      "user": "testuser2",
+      "password": "password456",
+      "mode": "archive",
+      "folderFilter": {
+        "include": ["Work/*", "Archive/*"],
+        "exclude": ["*/Drafts"]
+      },
+      "messageFilter": {}
+    }
+  ]
+}