Integrate Glean Gradle plugin

[cketti]

See https://mozilla.github.io/glean/book/user/adding-glean-to-your-project/kotlin.html#setting-up-metrics-and-pings-code-generation

Note: The Python envs plugin tries to add a dependency repository to the (sub)project. This is currently not allowed in our project. We'll have to change the build configuration or use a modified version of the Gradle plugin.