💥 Use ExternalStorageReference protobuf message for payload references

[jmaeagle99]

What was changed

• Store and retrieve payload references as ExternalStorageReference protobuf message encoded with protojson.
• Allow backward compatible retrieval of prerelease plain json encoding.
• Embed ExternalStorageReference proto message definition in sdk-go until api-go contains the same definition. Generate structure and data such that wire format and data are exactly the same as would be in api-go.

💥 Breaking Change

Workflow history that contains an external storage reference that is encoded using the proto-JSON converter will likely not be runnable on workers using prior versions of the SDK due to this format change.

Why?

• Standardize on protobuf message for encoding storage references.

Checklist

1. How was this tested: Existing tests, compat tests, and new tests
2. Any docs updates needed? No

[yuandrew]

For reference, this is the API PR: temporalio/api#772