Allow unit return type for suspend functions

[oguzcelik]

Summary

This pr aims to fix the lint rule for retrofit usage. Solves this issue

For suspend functions it's a valid case to return Unit.

Requirements (place an x in each [ ])

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[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @oguzcelik to sign the Salesforce Inc. Contributor License Agreement.

[ZacSweers]

Let's make this more intentional. Could we either add an opt-in annotation like @AllowUnitResponse with a doc on it explaining contexts where it makes sense? Blanket disabling this feels like the wrong move when the goal is to make developers more intentional.

[oguzcelik]

I followed other examples of how to make the test recognise this import. Don't know if there is a better way.

[oguzcelik]

Marked as Binary retention as we don't need this at runtime.

[oguzcelik]

Would you want to make the docs more explicit ? Something like:

/**
 * Annotation to allow suspend Retrofit functions to return Unit.
 * 
 * This annotation is used to indicate that a suspend Retrofit function is allowed to have 
 * a return type of [Unit]. This might be necessary for certain HTTP methods where the 
 * response body is not significant. For instance:
 * 
 * - Methods like `@POST`, `@PUT`, or `@DELETE` may logically return `Unit` if 
 *    the client is only concerned with the success/failure of the operation and not the 
 *    actual response body.
 * 
 * Invalid Use Cases:
 * - `@GET`: Typically, `@GET` methods should return a meaningful response object as 
 *    they are intended to retrieve data.
 */