[FME-12059] SDK_UPDATE with metadata

[ZamoraEmmanuel]

JavaScript commons library

What did you accomplish?

How do we test the changes introduced in this PR?

Extra Notes

[EmilianoSanchez]

If metadata is only used inside the if condition, we should refactor it as:

if (readiness) {
  const metadata: SdkUpdateMetadata = {
    type: SdkUpdateMetadataKeys.SEGMENTS_UPDATE,
    names: []
  };
  readiness.segments.emit(SDK_SEGMENTS_ARRIVED, metadata);
}

[EmilianoSanchez]

Idem for splitChangesUpdater

[EmilianoSanchez]

In my opinion this enum should be removed and string constants should be used instead, because the SdkUpdateMetadata type in splitio.d.ts already defines the type field as a string literal union.

So, you can remove the enum and define the following constants in the readiness/constants.ts file:

...

// SdkUpdateMetadata types:
const FLAGS_UPDATE =  'FLAGS_UPDATE';
const SEGMENTS_UPDATE = 'SEGMENTS_UPDATE'

and reuse them across the code.

Using string literal unions rather than enum or const enum is a recommended practice for several reasons. For example, the output built code is smaller, const enum is incompatible with some build setups, etc.