RUMM-1154 User can add attributes to auto instrumented RUM Resources

[ncreated]

What and why?

📦 This PR adds rumResourceAttributesProvider API, which makes it possible to add extra attributes to RUM Resources collected automatically:

public func setRUMResourceAttributesProvider(
   _ provider: @escaping (URLRequest, URLResponse?, Data?, Error?) -> [AttributeKey: AttributeValue]?
)

The provider block is called for each URLSessionTask intercepted by the SDK. The implementation should look into task information and return custom attributes for RUM Resource, e.g. send redacted response headers or decode response body and send its additional context. This block is also called for RUM Errors created for resources failed on transport.

This is a counterpart of DataDog/dd-sdk-android#520 introduced in dd-sdk-android.

How?

The implementation starts with passing the provider block through configuration object to URLSessionRUMResourcesHandler as we do for other stuff. URLSessionInterceptor is extended with additional method: taskReceivedData(task:data:) which is called from two places:

• from URLSesionSwizzler - for requests sent with completion block,
• from DDURLSessionDelegate - for requests sent with no completion block.

About the public API, I considered few other choices:

• adding the provider parameter to DDURLSessionDelegate.init() - this would require also exposing it in URLSessionInterceptor (for no-delegate usage including Alamofire extension),
• adding the provider parameter to existing .trackURLSession(firstPartyHosts:) as both are mutually related - this would make the discovery of this new feature harder (the doc comment for .trackURLSession is already huge).

The proposed API should keep things simple: the feature can announced by its API name and the mutual relationship with trackURLSession is solved by installing misconfiguration warning.

All related unit tests pass on iOS 14.3, 13.4, 12.4 and 11.1.

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

[xgouchet]

LGTM 👍