iOS - Allow changing of User-Agent

[hactar]

MapLibre allows setting additional headers to be sent in network requests via MLNNativeNetworkManager.sharedManager.sessionConfiguration. Here a user could set up a User-Agent to be sent during requests. Unfortunately, in http_file_source.mm, we set a User-Agent without checking if a user might have set one too.

This prevents users from being able to set a User-Agent at all. In my use case for example, I have to talk to a service which expects a certain User-Agent, and currently I have no way to get MapLibre to use the required User-Agent.

This PR resolves the issue by checking if a custom User-Agent has been set in MLNNativeNetworkManager.sharedManager.sessionConfiguration first - if one is set, then that one is used, else MapLibre defaults to its own.

[louwers]

Sounds reasonable.

If you want a release for this right away feel free to update platform/ios/VERSION and platform/ios/CHANGELOG.md (6.4.0).

[github-actions]

Bloaty Results (iOS) 🐋

Compared to main

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%     +56  [ = ]       0    TOTAL

Full report: https://maplibre-native.s3.eu-central-1.amazonaws.com/bloaty-results-ios/pr-2293-compared-to-main.txt

[hactar]

@louwers Changelog and version updated. Thanks.