Options for generating manifests are unclear when each should be used

[CraigSiemens]

Use case

From what I can tell, there's two different properties available that sound like they enable manifest generation

• ApolloCodegenConfiguration.OperationManifestConfiguration.generateManifestOnCodeGeneration
• itemsToGenerate when calling ApolloCodegen.build passing a ApolloCodegen.ItemsToGenerate

When calling ApolloCodegen.build directly, it's unclear from those properties whether they should be used together or if just one should be used since they're both passed into build.

Describe the solution you'd like

From trial and error I think that generateManifestOnCodeGeneration is only used when calling the generate command on the CodegenCLI. It'd be nice if the documentation was clearer that it only applies when using CodegenCLI.

Ideally there would be one way to enable generating manifests so the same ApolloCodegenConfiguration passed to CodegenCLI and ApolloCodegen.build would produce the same results.

[BobaFetters]

I agree this is a little confusing as to what the options do, although I think the right solution here would be to add a parameter for use with the Codegen CLI to trigger manifest generation and remove the option from the configuration (deprecate initially) because the overall goal is to give users flexibility with manifest generation and currently the ApolloCodegen.ItemsToGenerate passed into the build method provide that because you can choose for any given run of codegen whether you want a new manifest, while the current codegen configuration forces it to generate everytime. So switching to a cli parameter would provide the same flexibility for CLI to choose when to generate a manifest, and if you wanted to generate one everytime then it's easy enough to just pass the parameter everytime without forcing it.

@AnthonyMDev @calvincestari thoughts?

[AnthonyMDev]

I think that approach makes sense to me! Seems like an improvement on what I originally designed here.