Update clients to interact with (new) server encrypted databases (#2113)

This patch enables client to send encryption key header for encrypted
databases. The header we are required to send is
`x-turso-encryption-key`.

To connect with encrypted database, either remote or offline write, you
may use the builder pattern to provide the encryption context. Ex.:

```rust
let encryption = if let Ok(key) = std::env::var("LIBSQL_ENCRYPTION_KEY") {
        Some(EncryptionContext {
            key: EncryptionKey::Base64Encoded(key),
        })
    } else {
        None
    };
let db_builder =
        Builder::new_synced_database(db_path, sync_url, auth_token).remote_encryption(encryption);
```

This patch also comes with a full example:
https://github.com/avinassh/libsql/blob/5b95a1928b821855fced3880caf7871ce032c58b/libsql/examples/encryption_sync.rs