misc: add mounted_secrets parameter with defaulting to old behavior

[isidentical]

specify which secrets you want to mount and which not

class MountedSecretsApp(fal.App, keep_alive=300, max_concurrency=1):
    machine_type = "XS"
    mounted_secrets = ["MOUNTED_TEST_SECRET"]

    @fal.endpoint("/")
    def check_secrets(self) -> SecretsOutput:
        return SecretsOutput(
            has_mounted="MOUNTED_TEST_SECRET" in os.environ,
            has_not_mounted="NOT_MOUNTED_SECRET" in os.environ,
        )

by default we mount to mounted_secrets = ["*"].