dataconnect: demo: build.gradle.kts: add ability to specify the Data Connect emulator executable to use

[dconeybe]

Enhance the Data Connect demo's Gradle build configuration by introducing a new option to explicitly define the path to the Data Connect emulator executable. This provides flexibility for developers who may need to use a specific version or a locally-built emulator binary instead of relying on the one automatically managed by the Firebase CLI.

Highlights

• Emulator Executable Path: Added ability to specify the path to the Data Connect emulator executable via a new Gradle property (dataConnect.demo.dataConnectEmulatorExecutable). This allows overriding the default emulator used by the firebase command.
• Environment Variable: When the custom emulator path is provided, the build script now sets the DATACONNECT_EMULATOR_BINARY_PATH environment variable before executing the firebase command, instructing it to use the specified binary.
• Gradle Property Naming: Updated the Gradle property names used in the demo build from dataConnect.minimalApp.* to dataConnect.demo.* for consistency.