dataconnect: fix gradle plugin to gracefully handle schemaExtensionsOutputEnabled not being explicitly specified

[dconeybe]

The schemaExtensionsOutputEnabled knob of the Data Connect gradle plugin was always meant to be optional; however, it was accidentally made to be required, resulting in the following unnecessary error if it was not explicitly specified:

A problem was found with the configuration of task ':firebase-dataconnect:connectors:runDebugDataConnectEmulator' (type 'DataConnectRunEmulatorTask').
  - In plugin 'com.google.firebase.dataconnect.gradle.plugin.DataConnectGradlePlugin$Inject' type 'com.google.firebase.dataconnect.gradle.plugin.DataConnectRunEmulatorTask' property 'schemaExtensionsOutputEnabled' doesn't have a configured value.
    
    Reason: This property isn't marked as optional and no value has been configured.
    
    Possible solutions:
      1. Assign a value to 'schemaExtensionsOutputEnabled'.
      2. Mark property 'schemaExtensionsOutputEnabled' as optional.

This PR correctly makes it optional, defaulting to false.

[github-actions]

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