Add a Gradle plugin configuration DSL (CLI path override, per-variant toggles)

[ioannisj]

Extracted from PostHog/posthog#43012.

Problem

The plugin registers no extension, so there's no way to configure it. The CLI path is hardcoded to posthog-cli, and the only skip logic is !isMinifyEnabled; per-variant control is still a // TODO in the plugin.

Scope

• Add a posthog { } extension for forcing the CLI binary path, enabling or disabling per build type and per variant, an autoUpload toggle, allow-failure, and debug mode.