fix: Add namespace for AGP 8.0+ compatibility

[Keith-wright]

Summary

• Adds namespace declaration to pusher_beams_android for Android Gradle Plugin 8.0+ compatibility

Problem

Starting with AGP 8.0, the namespace property must be declared in build.gradle rather than relying on the package attribute in AndroidManifest.xml. Without this, builds fail with:

Namespace not specified. Specify a namespace in the module's build file.

Solution

Added namespace 'com.pusher.pusher_beams' to the android block in packages/pusher_beams_android/android/build.gradle.

Fixes

• Fixes Flutter 3 Error Namespace not specified #58
• Fixes Missing Namespace in latest AGP #60
• Fixes Missing Namespace in latest AGP #61