Add ODoH cryptographic routine implementation (#44)

### Motivation:
This change implements the core cryptographic routine for Oblivious DNS
over HTTPS (ODoH) as specified in RFC 9230.

### Modifications:
- Added complete `ODoH.Routine` struct with client-side and server-side
cryptographic operations
- Added comprehensive usage documentation with complete client and
server examples
- Created full roundtrip test validating end-to-end
encryption/decryption flow

### Result:
The library now provides RFC9230 compliant ODoH cryptographic
functionality.

Author: yigityazicilar