message: add backward compatibility for AccountProviderConfig
  * add backward compatibility for AccountProviderConfig
  add serde default for AccountProviderConfig's field named from_env
  * add skip_serializing_if to secret_file
  add skip_serializing_if to secret_file
diff: @@ -17,6 +17,7 @@ pub struct AccountProviderConfig {
  pub account_dir: Option<PathBuf>,
  /// Path to the secret file storing the private key.
  + #[serde(skip_serializing_if = "Option::is_none")]
  #[clap(
  long = "secret-file",
  help = "file path of private key",
  @@ -25,6 +26,7 @@ pub struct AccountProviderConfig {
  pub secret_file: Option<PathBuf>,
  /// Read private from env variable `STARCOIN_PRIVATE_KEY`.
  + #[serde(default)]
  #[clap(long = "from-env")]
  pub from_env: bool,
  