Improvement: Enable `UseCachedSize` in proto marshal to eliminate redundant size computation

[rs-unity]

The proto message size is currently computed twice:

1. Once before marshalling.
2. Again during the marshalling call itself.

In high-throughput workloads, this duplicated computation is expensive and unnecessary.

By enabling UseCachedSize on MarshalOptions, we can reuse the size calculated immediately before marshalling, avoiding the redundant second call to proto.Size.

Impact

In our application, the extra proto.Size call accounted for ~12% of total CPU time. Removing it results in a significant performance improvement while preserving correctness.

Proposed Change

• Enable UseCachedSize in MarshalOptions to eliminate duplicate size calculations. See encoding/proto: enable use cached size option #8569