Add the ability to log Flow callout requests and responses

[jongpie]

Flows has the ability to make HTTP callouts with External Services, similar to the Apex classes HttpRequest, HttpResponse, and Http that can be used to make callouts.

Ideally, Nebula Logger should provide a way to log the Flow callout request and response, similar to the Apex instance methods LogEntryEventBuilder.setHttpRequestDetails() and LogEntryEventBuilder.setHttpResponseDetails() (see discussion #632). However, I haven't use this functionality in Flow, so I'm not sure how exactly this works in Flow, nor what the data types are of the Flow callout request & response. It might "just work", but it might require some changes to Nebula Logger's 3 invocable logging classes - FlowLogEntry, FlowLogRecordEntry, and FlowCollectionLogEntry - to support this.