RUMM-883 Add support for 3rd party networking libraries (including Alamofire)

[ncreated]

What and why?

📦 This PR makes it possible to add integrations with 3rd party networking libraries (including Alamofire).

How?

Our network interceptor is pretty straightforward and follows the interception model in Alamofire:

• modify request;
• process task creation;
• process task metrics;
• process task completion (with optional error).

By polishing its interface and making the URLSessionInterceptor public, now it is possible to use our SDK with Alamofire by implementing custom AF's EventMonitor (for task start, metrics and completion) and AF's RequestInterceptor (for request modification).

Implementations of DDEventMonitor and DDRequestInterceptor are provided in Sources/DatadogExtensions/Alamofire/.

Those implementations are delivered in DatadogSDKAlamofireExtension pod:

pod 'DatadogSDKAlamofireExtension'

import DatadogAlamofireExtension
import Alamofire

let alamofireSession = Session(
   interceptor: DDRequestInterceptor(),
   eventMonitors: [DDEventMonitor()]
)

DatadogAlamofireExtension for Carthage and SPM

It is not yet possible to distribute DatadogAlamofireExtension as SPM package, because dynamic frameworks are still broken in SPM. Doing so, raises the compiler warning of duplicated symbols for Datadog and Kronos (as those two are (in-)directly linked to the app by both Datadog and DatadogAlamofireExtension dependencies).

Distributing DatadogAlamofireExtension through Carthage is very inconvenient and may be error-prone for non-Alamofire users, as both DatadogAlamofireExtension.framework and Alamofire.framework will appear in Carthage/Build/iOS folder. This would require additional explanation in Carthage installation steps for the SDK.

Note on DatadogSDKAlamofireExtension.podspec

Instead of creating separate podspec, I've also considered also adding AlamofireExtension subspec to DatadogSDK.podspec or creating DatadogSDKExtensions/Alamofire spec. Neither of this seems valid, as it doesn't scale for other integrations and eventual SPM / Carthage support.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference

[ncreated]

As discussed on daily, I created DatadogSDKAlamofireExtension pod which provides the helpers. The description of this PR was updated with more details.

[ruthnaebeck]

Docs review

[ruthnaebeck]

👍 for docs