fix: add X-WS-* header detection and attachment test

- Detect X-WS-* headers (e.g., X-WS-Attachment-UUID) as MIME headers
  to ensure attachment headers like Content-Type and Content-Disposition
  get normalized properly
- Add test case for attachment header normalization
- Bump version to 0.5.0

The normalization fixes malformed continuation lines in attachment
headers generated by Infomaniak webmail, where lines like
"name=file.pdf" are missing the required leading whitespace.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "rs_pop_imap_importer"
-version = "0.3.0"
+version = "0.5.0"
 edition = "2024"
 
 [lib]