Add support for Gradle Plugin DSL (plugins {})

Description
Currently, the com.appdynamics:appdynamics-gradle-plugin can only be applied via the legacy
buildscript { ... } + apply plugin: syntax.
When trying to apply it using the modern declarative Gradle Plugin DSL (introduced in Gradle 7+):
```
plugins {
id "com.appdynamics.appdynamics-gradle-plugin" version "22.2.2"
}
```
Gradle fails with the following error:
`Could not apply requested plugin [id: 'com.appdynamics.appdynamics-gradle-plugin', artifact: 'com.appdynamics:appdynamics-gradle-plugin:22.2.2'] as it does not provide a plugin with id 'com.appdynamics.appdynamics-gradle-plugin'. This is caused by an incorrect plugin implementation. Please contact the plugin author(s).`