Add jsonName and json struct tag support

[NukaCody]

Issue #326, if available:

Description of changes:

If you specify jsonName("somevalue") trait in your smithy model, then assume you want a json struct tag.

I think that's the most intuitive approach to take this but definitely open to other ideas. Something like you have to specify in smithy-build.json some value such as useJsonStructTag: true. But may be more work for not much reward.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[NukaCody]

Also, I have no clue what the letters in writer.write("$L $P \json:\"$L\"", memberName, memberSymbol, jsonTag); mean lol. I tried $C and kept running into a InvocationTargetException so I went with L and it worked

[jasdel]

We've been hesitant to add struct tags to the API types. Mainly because the SDK strives to ensure serialization in an implementation detail of the API and SDK. An API could always change it's protocol and that would put the struct tags at risk of breaking. The public types purposely do not expose these.

We're happy to continue this discussion and hear your use case about the need for this feature. Also it's important to note that API protocol serialization isn't Go JSON standard especially in cases such as byte slices and time.

[NukaCody]

I can understand that. If I capture it right, looking through an example. EC2RunInstances and it's Input make no assumption on protocol. It's not till invokeOperation and I think more specifically HTTP semantics start here

Our use case is a little bit different. We have a folder structure like this:

.
├── model
│   ├── build/smithyprojections/model/source/go-codegen
└── externalLib # Both use model
└── infra # Both use model

Our model is used in two different contexts, one as supplying an actual API in the traditional sense of an SDK talking to an API endpoint in Infra. Another as a sort of "shared type" folder. In externalLib we have to build a custom marshaller/unmarshaller that transforms a specification (that we can't control) into cloudformation. The jsonName tag makes writing the custom marshaller easier.

I think jsonName() is also a two-way door. If we need to backtrack, we can always Find and Replace jsonName(*) to "" then regenerate the model. Although I do see how that would break externalLib's marshalling if we did that. I think that's a risk we are willing to accept though.

I'm a bit smooth brain and didn't follow this:

Also it's important to note that API protocol serialization isn't Go JSON standard especially in cases such as byte slices and time.

[jvschneid]

Consider using optionals here instead of all these conditionals, something like:

writer.writeInline("$L $P", memberName, memberSymbol);

member.getMemberTrait(model, JsonNameTrait.class).map(JsonNameTrait::getValue)
      .ifPresent(jsonName -> writer.writeInline(" `json:\"$L\"`", jsonName));
                        
writer.write("");

[lucix-aws]

I'm going to close this - this should be handled via protocol generation, specifically we'll get json protocol support (including jsonName) by addressing #458.