Provide details about support for optional API features in ImplementationMetadata

[kriswest]

Enhancement Request

Use Case:

One of the goals of the FDC3 standard is to allow apps from vendors to interoperate with each other in a standardized fashion, via a compliant implementation of the Desktop Agent API. However, as the standard can contain optional features (indicated via the SHOULD or MAY keywords) it would be helpful to define, as part of the standard, how support for these features can be queried or tested.

Proposal

The fdc3.getInfo() call added in 1.2 returns an ImplementationMetadata object that contains information about the FDC3 standard version and Desktop Agent implementation and would seem an ideal place to also insert details of which optional features are supported, e.g..

enum OptionalFeatures {
   "USER_CHANNEL_PICKER": "UserChannelPicker",
   ... // further optional features
}

public interface ImplementationMetadata {
  fdc3Version: string;
  provider: string;
  providerVersion?: string;
  optionalFeatureSupport: Record<OptionalFeatures, boolean>;
}

Usage example:

//Check for an optional feature
if (fdc3.getInfo) {
  let implementationMetadata = await fdc3.getInfo();
  if (!implementationMetadata.optionalFeatureSupport[OptionFeatures.USER_CHANNEL_PICKER]) {
      //render our own channel picker as the Desktop Agent doesn't provide one
  }
} else {
  //fdc3 version < 1.2 or non-compliant implementation
}

Additional information

Optional features can and should be added to the enumeration alongside the work on #484 to formalize compliance information throughout the standard