fix: rename jsonCodec to avoid overriding gRPC default proto codec

[envestcc]

Summary

Renamed the jsonCodec from "proto" to "ioswarm-json" to avoid globally
overriding gRPC's default protobuf codec.

Problem

The previous implementation registered a JSON codec with the name "proto", which
overwrote gRPC's default protobuf codec globally. This caused all gRPC services in
the same process to use JSON encoding instead of protobuf, breaking compatibility
with standard gRPC clients.

Solution

Changed the codec name from "proto" to "ioswarm-json":

• Maintains JSON serialization for ioswarm internal communication
• No longer interferes with other gRPC services using protobuf
• Must be paired with the same change on the coordinator/server side

Changes

• codec.go: Changed jsonCodec.Name() return value from "proto" to
  "ioswarm-json"
• Updated comments to clarify the purpose of using a distinct codec name

Related

This PR should be merged together with the coordinator fix in iotex-core.