@W-16589742: [iOS] REST wrappers for select SFAP APIs

[JohnsonEricAtSalesforce]

🧱 Draft: Only one of the four endpoints is included. We'll review that for a quick look, then add the remaining three 🚧

This adds REST API clients for select sfap_api endpoints to match the Android version in forcedotcom/SalesforceMobileSDK-Android#2644

Only one of the four is included in the draft status. Here's a code sample from how I tested in iOSNativeSwiftTemplate's AppDelegate.

    /// The generation id from the `sfap_api` `generations` endpoint
    private var generationId: String? = nil
    
    /// Fetches generated text from the `sfap_api` "generations" endpoint.
    private func generateText() async {
        do {
            guard let userAccountCurrent = UserAccountManager.shared.currentUserAccount, let restClient = RestClient.restClient(for: userAccountCurrent) else { return }
            
            // Guards.            
            let generationsResponseBody = try await SfapApiClient(
                apiHostName: "dev.api.salesforce.com",
                modelName: "sfdc_ai__DefaultGPT35Turbo",
                restClient: restClient
            ).fetchGeneratedText(
                "Tell me a story about an action movie hero with really, really cool hair."
            )
            
            self.generationId = generationsResponseBody.generation?.id
            print("SFAP_API-TESTS: \(String(describing: generationsResponseBody.generation?.generatedText))")
            print("SFAP_API-TESTS: \(String(describing: generationsResponseBody.sourceJson))")
        } catch let error {
            SFSDKCoreLogger().e(
                SfapApiClient.classForCoder(),
                message: "Cannot fetch generated text due to an error with message '\(error.localizedDescription)'.")
        }
    }

[bbirman]

Why is this getting a RestClient based on the current user vs using the RestClient from init?

[bbirman]

if you don't want to rename it, I think you could also do guard let modelName { here

[bbirman]

I'm a little confused with the error handling, why does the error need to be caught and re-thrown?

[bbirman]

Why does this use NSError? Was there an issue with Swift's error type?