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

[nick4598]

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.
#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)

[JulijaRamoskiene]

Rplied to your question about exporter: #172 (comment)

To answer your question about Exporter, yes, we do set up exporter manually.
We have our custom Exporter: https://dev.azure.com/bentleycs/iModelTechnologies/_git/imodel-transformations?path=/packages/imodel-transformations-local/src/Exporter.ts
Which is initialized and passed to transformer here: https://dev.azure.com/bentleycs/iModelTechnologies/_git/imodel-transformations?path=/packages/imodel-transformations-local/src/transformers/TransformerFactory.ts

[nick4598]

PR linked will be to untangle initializing and calling processChanges, so consumers could run anything they wanted in between calling initialize() and process() on the transformer