[ios][macos] Add MGLNetworkConfiguration.sessionDelegate

[halset]

This is an attempt to solve #11888 by expanding MGLNetworkConfiguration with a property to set a custom NSURLSessionDelegate.

My main motivation for doing this is to be able to use tile services that uses HTTP Basic Authentication.

This is a new version of #13491 as that was made before MGLNetworkConfiguration went public.

[julianrex]

Since sessionWithConfiguration:delegate:delegateQueue: keeps a strong reference to the delegate, I think we'll need to call invalidateAndCancel or finishTasksAndInvalidate in the destructor.

[halset]

Sounds good. I can add a destructor to HTTPFileSource::Impl and call invalidateAndCancel on theNSURLSession there, but how can I test it? Is there a way to tear down the stack? I have tried to stop the app, but the destructor does not seem to be called.

[julianrex]

Apologies for the delay in responding - I believe we'll need to create a test for this. There are cpp tests for HTTPFileSource and OnlineFileSource which we might be able to take advantage of, but I think we'll need to create an integration test for this.

[julianrex]

cc: @tmpsantos for an interesting testing scenario (platform <-> core).

[julianrex]

Added: #15592

[julianrex]

Thanks for this PR @halset - we will discuss and review!

/cc @chloekraw @zugaldia

[halset]

@julianrex There is a merge-conflict in CHANGELOG.md, but I will await your discussion/review.

[hectorddmx]

Hi @halset, one question, would this help to recreate the session with a new configuration?
Currently we're using bearer tokens in iOS, which expire in 15 minutes, and I'm able to call vector tiles from my custom endpoint by setting MGLNetworkConfiguration.sharedManager.sessionConfiguration.httpAdditionalHeaders = ["accessToken": newToken]

But after the token is refreshed, I'm getting auth issues since I'm not able to recreate the session with a new configuration.

[julianrex]

Requires #15592

[patrickkempff]

any news on this, or how to set a custom header to the request done by mapbox? On Android it is easy, on iOS somehow not..

[julianrex]

This PR is still valid, but needs to be renewed/reviewed in light of recent changes.

/cc @jmkiley

[julianrex]

Closing in favour of mapbox/mapbox-gl-native-ios#228