feat(rust): add dt parameter to encryption response

[coreyhn]

This change makes it so the cast_as value from the encryption configuration is returned as the dt parameter in all encryption responses. The data type parameter makes it so encrypted data can be type casted without requiring access to the original configuration. This will allow the cipherstash/protectphp library to have automatic data type detection during encryption (with optional overrides) and data type casting during decryption when using this library, for example:

$encrypted = Protect::encrypt('users.balance', 42);

echo $encrypted;
// {"k":"ct","c":"mBbJ_{U+!@oO9P*hGG5rslSWF6d2...","dt":"small_int","hm":"89bd7c0deb763305bf26212aa96f...","ob":["9930303030303030c68b2822197a..."],"bf":null,"i":{"t":"users","c":"balance"},"v":2}

$decrypted = Protect::decrypt($encrypted);

echo $decrypted;
// 42

The strum crate was added for CastAs enum string serialization instead of relying on serde because it provides better performance for this use case. Rather than using serde to serialize the entire enum to JSON just to extract the string representation for a single parameter, strum's dedicated Display derive macro allows direct string conversion with snake_case formatting while maintaining separate control over JSON serialization.