Couchbase: Use big endian byte order for opaque field
The opaque field was using ENC_LITTLE_ENDIAN, causing Wireshark to display values with reversed byte order compared to Couchbase client SDK logs. For example, the SDK would show 0x0032a31e while Wireshark showed 0x1ea33200 for the same value.
Change to ENC_BIG_ENDIAN to match:
- Network byte order standard
- All other multi-byte header fields in the dissector
- The memcache dissector implementation
- Couchbase SDK display format
This allows easy correlation of opaque values between Wireshark captures and SDK logs.