Provide hook after transformer initializes to allow consumers to run some code.

Context

NOTE: Not urgent or blocking! There are already currently working workarounds in SVCs side for this. SVCs team has a function manualInitialize which is used mainly for: this._startChangesetIndex = this.synchronizationVersion.index + 1;.

We should support SVCs team desire to determine the startChangesetIndex so that they do not have to have a manualInitialize which has to set a private property _isSynchronization on the core transformer.

Potential Options

Provide a hook onTransformerInitialized which lets consumers run code right after the transformer is initialized and before it starts processing changes. Consumers would pass this function as part of the transformer options.
Adding method to transformer to get the startchangesetindex?

Maybe on red arrow could add? Or somewhere in this function above

Concerns

I raised a concern on the below PR that I think could be relevant. https://github.com/iTwin/imodel-transformer/pull/172

One other concern I have is, does the transformation services team ever setup the exporter manually before you call the transformer? Currently you'll need to add your custom changes before you call IModelTransformer.initialize(), which I personally ran into some trouble when trying to allow myself to do that in the tests (can look at timelinetestutil to see what changes I had to make to support that)

iTwin / imodel-transformer