Core Library for configuring an audience to modify a model

[bryan-strassner]

Clear and concise description of the problem

After discussion I was asked to open an issue/feature request for the following:

I have a scenario where I would like to be able to configure that a model or some model properties are only included if the correct audience is configured for the compilation. Then, when triggering a compile, allow for options to be configured to enable certain audiences for that compilation.

I have implemented (with help of course) a localized decorator `@audience() to do this but believe that a more fully developed solution that is integrated into the core library would be useful.

My implementation looks like this:

/**
 * Applies audience-based filtering to a model, model property or operation. Requested audiences are
 * read from the `options.networkcloud.audiences` entry in the active TypeSpec configuration. If no config value
 * is provided, no audiences are enabled and all annotated items are removed.
 *
 * Configuration example:
 *   options:
 *     mynamespace:
 *       audiences:
 *         values:
 *           - internal
 *
 * Usage examples:
 *    // A model, targeting a single audience
 *   @audience("internal")
 *   model InternalModel { ... }
 *
 *   // Targeting multiple audiences
 *   @audience("internal", "external")
 *   model InternalOrExternalModel { ... }
 *
 *   // An individual model property
 *   model SomeModel {
 *     @audience("internal")
 *      someProperty: string;
 */
extern dec audience(
  target: Union | Model | ModelProperty | Operation,
  ...audienceTargets: valueof string[]
);

In my case, I want to use the same emitter to produce both an internal (service) and external (user) facing representation of my API - both in terms of an OpenAPI schema, and for a Go SDK.

In discussion, this concept could be adapted so that it has applicability to the model as a whole - reflecting in all emitters but could be designed to apply to only certain emitters, or classes of emitters.

Checklist

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[markcowl]

@witemple-msft per discussion, please evaluate