Send resource diff in the BotEvent

[reshmakh]

We're starting to see more use cases for Bots where the author wants to do something when a specific change happens.

The primary example is a customer, where they want to perform some action when a ServiceRequests's status is changed from one value to another. Example

There is a workaround right now to call readHistory , but this is wasteful because you often only need the immediately preceding entry. There's also a potential race condition if the resource is frequently updated.

There are couple different ways we could represent the "diff":

Just add another property called BotEvent.previousInput that contains the entire object before the subscription was fired (i.e. calls readHistory on the user's behalf)

Add a BotEvent.diffs property that has a list of JSON patches containing all the changes made (seems complicated, but potentially less data?)

Add a BotEvent.diff property which is a JS object that contains only the fields that changed (weird?)