Add ledger webhooks support

[ryanwjackson]

Is your feature request related to a problem? Please describe.
If a user deletes a resource in the ledger (e.g. QBO) when it already exists in rails_app as a LedgerResource, future syncs will fail as the ledger_id is no longer valid.

Describe the solution you'd like

• Notify the provider that the delete has occurred.
• Remove the ledger_id on the LedgerResource so future syncs will succeed as expected.

Questions

• Do we retry any current failures due to a race case of a sync failing before the webhook was received and processed?
• What are other use cases besides deletion?
• Should this functionality be built into the library somehow?

@mattgmarcus @jozefvaclavik

[mattgmarcus]

What are other use cases besides deletion? is a good question. Perhaps we can build this out with different events one at a time. Deletion is probably both the hardest and the most important to handle

[jozefvaclavik]

We don't fail currently. Thats bit due to calling just upsert and not update/create directly.

Upsert tries to find object based on ledger_id, if that succeed, we do update; if that fails, we do create.

So if user removes object in QBO, we will create it again.

[ryanwjackson]

Ah, that's interesting.  Not sure we want that behavior. @mattgmarcus thoughts?