Add Python Attribute Accessors and Cross-Library Compatibility for PkEncryption

[TrevisGordan]

This PR adds missing Python attribute accessors and improves compatibility with python-olm by enabling cross-library encryption and decryption tests.

Details:

• Adds #[pyo3(get)] annotations to the Message struct fields (ciphertext, mac, ephemeral_key), making these attributes accessible from Python.
• Implements a Message constructor in Rust (via #[new]), allowing Python code to create Message instances directly.
• Introduces tests to verify that attribute access works as intended.
• Adds integration tests to ensure PkEncryption is compatible with python-olm. Specifically:
  • Encrypt with Olm and decrypt with Vodozemac.
  • Encrypt with Vodozemac and decrypt with Olm.

Next Steps:

To achieve seamless compatibility, the user will need to Base64-encode Vodozemac message attributes (ciphertext, mac, ephemeral_key) before passing them to Olm. For example:

vodo_encryption = PkEncryption.from_key(public_key)
vodo_message = vodo_encryption.encrypt(PLAINTEXT).encode())

vodo_mac = base64.b64encode(vodo_message.mac).decode('ascii')
vodo_ciphertext = base64.b64encode(vodo_message.ciphertext).decode('ascii')
vodo_ephemeral_key = base64.b64encode(vodo_message.ephemeral_key).decode('ascii')

A future enhancement could add a Rust helper function to handle this encoding automatically, such as:

(ciphertext, mac, ephemeral_key) = encryption.encrypt(json.dumps(media_body).encode()).to_base64()

Also unpad_base64_encode and pad_base64_decode functions.
This would streamline the process and reduce the amount of Python-side encoding logic required.