[minor][Feature]: Provide public api to set custom headers for CIAM requests

[antrix1989]

Proposed changes

1. Configure headers before calling the API (for example, at app launch):

    func application(_ application: UIApplication,
                     didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        
        MSALNativeAuthHTTPConfig.customHeaders = [
            "customer_header_1 ": "value_1",
            "customer_header_2 ": "value_2",
            "x-client-DM": "OOOOO" // Will be ignored: conflicts with a header set internally by the MSAL SDK.
        ]
        

        return true
    }

2. Call the SDK APIs -- headers will be added to all network requests

Type of change

• Feature work
• Bug fix
• Documentation
• Engineering change
• Test
• Logging/Telemetry

Risk

• High – Errors could cause MAJOR regression of many scenarios. (Example: new large features or high level infrastructure changes)
• Medium – Errors could cause regression of 1 or more scenarios. (Example: somewhat complex bug fixes, small new features)
• Small – No issues are expected. (Example: Very small bug fixes, string changes, or configuration settings changes)

Additional information