grpc/codegen: allow overriding proto json_name

[ilCollez]

Summary

• extend the DSL with Meta("proto:tag:json", ...) so designers can override the json_name emitted in generated protobuf definitions
• plumb the metadata through grpc/codegen/protobuf.go, emitting [json_name = "..."] on both regular fields and oneof members when present
• update the DSL documentation to mention the new metadata key and add unit tests that cover the option for structs and oneofs

Motivation

Some downstream consumers rely on protobuf JSON descriptors whose field names must diverge from the default lower_snake_case. Prior to this change there was no way to control the descriptor in Goa-generated protos without patching the generated files. The new metadata brings parity with the existing struct-tag overrides and keeps designs self-contained.

Example

DSL:

var Item = Type("Item", func() {
    Field(1, "ID", String, func() {
        Meta("proto:tag:json", "identifier")
    })
})

Generated .proto fragment

message Item {
    string id = 1 [json_name = "identifier"];
}

Generated Go struct (.pb.go)

type Item struct {
    Id string `protobuf:"bytes,1,opt,name=id,proto3" json:"identifier,omitempty"`
}

[raphael]

Looks great, thank you!