Better support Gradle Configuration cache

Currently ([by inspection](https://github.com/google/secrets-gradle-plugin/blob/main/secrets-gradle-plugin/src/main/java/com/google/android/libraries/mapsplatform/secrets_gradle_plugin/SecretsPlugin.kt#L46)) this plugin reads the secrets file during configuration, which will result in a configuration cache miss when the contents of the secrets file changes

Could we use the lazy APIs (e.g. [fileContents](https://docs.gradle.org/current/javadoc/org/gradle/api/provider/ProviderFactory.html#fileContents(org.gradle.api.provider.Provider)) so this file is not read during configuration and therefore doesn't need to be a configuration cache input?