feat: Implement --ephemeral flag to prevent emulator data export

[christhompsongoogle]

The --ephemeral flag for the emulators:start command ensures that no data is exported when the emulators stop, even if --export-on-exit is specified or if the emulators:export command is used during the session.

Import functionality remains unaffected when using --ephemeral with --import.

Changes include:

• Modified controller.ts to check for options.ephemeral in exportOnExit and exportEmulatorData functions, preventing export and logging a message.
• Added integration tests in scripts/emulator-import-export-tests/tests.ts to verify:
  • No export occurs with --export-on-exit in ephemeral mode.
  • Data import works, but subsequent export is prevented in ephemeral mode.
  • The emulators:export command does not export data if emulators were started with --ephemeral.