Add Base64 Helper Functions for Vodozemac-Olm Interoperability

[TrevisGordan]

To streamline interoperability between Vodozemac and python-olm, introduce helper functions that handle Base64 encoding and decoding of message attributes (ciphertext, mac, ephemeral_key) directly in Rust. This will remove the need for manual encoding on the Python side and improve usability.

Current example:

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

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()

This would provide a clearer API, as different message encodings in use can be unclear to the user.

See: #15

[TrevisGordan]

With the help of the compatible base64_decode and base64_encode functions in Vodozemac, which handle base64 padding, I was able to directly implement this functionality in #15 .

The PR introduces a from_base64 class method and a to_base64 method, enabling straightforward usage as follows:

# Converting from base64
vodo_msg = Message.from_base64(
    olm_msg.ciphertext,
    olm_msg.mac,
    olm_msg.ephemeral_key,
)

# Converting to base64
ephemeral_key_b64, mac_b64, ciphertext_b64 = vodo_msg.to_base64()

Merging #15 would resolve this issue.