Use protojson marshaling for payload HTTP handler response

[jmaeagle99]

What was changed

Update the payloadHTTPHandler to write response as Protobuf JSON (according to codec server spec) instead of as plain JSON.

Why?

The codec server spec says that requests and responses should be interpreted with Protobuf JSON encoders.

The main observable difference is that plain JSON encoding would use snake_case for field names whereas Protobuf JSON uses camelCase for field names. This hasn't been an issue in the past because Payloads and Payload messages have property names that were only one word, thus the same in camelCase and snake_case.

With the introduction of the external_payloads field on the Payload protobuf message and the size_bytes field on the ExternalPayloadDetails protobuf message, these fields are being written as snake_case when serializing as plain json. The Web UI is expecting camelCase properties.

Brief testing with the Web UI and inspection of the code shows that this works. The Temporal CLI is already using NewRemotePayloadCodec from sdk-go, which is using protojson.Unmarshal on the response.

Checklist

1. How was this tested:

• Tested with existing WebUI.
• Inspect Temporal CLI for conformance.

1. Any docs updates needed? No