Add Android plugins support

[enfipy]

Objective

Adds initial Gradle Plugins support alongside Google AdMob Plugin. This will help developers create custom Android plugins with Rust wrapper to use Android features much more easily.

• Change creation of new Gradle project with help of RustEmbed and :app.
• Create extern methods to make it possible to talk from Java/Kotlin to Rust.
• Create JniSingleton structures to make it possible to call Plugin methods from Rust.
• Create Rust wrapper around extern Java methods.
• Add AdMobPlugin Android wrapper.
• Add AdMobPlugin Rust wrapper.
• Make it possible to specify the local Gradle Dependency Project in Cargo.toml.
• Provide an example of usage AdMobPlugin in a simple Macroquad project.
• Publish Crossbow Gradle project to Maven repository.
• Publish AdMobPlugin to the Maven repository.
• Update Docker image, add Gradle to it.