support anonymous credentials in rest level client

[qiaozha]

In confidential ledger and service fabric, we have cases like whether the credentials might be undefined, and we will nees tlsOptions in the options to do the authentication.

export default function ConfidentialLedger(
  ledgerBaseUrl: string,
  ledgerIdentityCertificate: string,
  credentials: TokenCredential,
  options?: ClientOptions
): ConfidentialLedgerClient;
export default function ConfidentialLedger(
  ledgerBaseUrl: string,
  ledgerIdentityCertificate: string,
  credentialsOrOptions?: TokenCredential | ClientOptions,
  opts?: ClientOptions
): ConfidentialLedgerClient {
  let credentials: TokenCredential | undefined;
  let options: ClientOptions;

  if (isTokenCredential(credentialsOrOptions)) {
    credentials = credentialsOrOptions;
    options = opts ?? {};
  } else {
    options = credentialsOrOptions ?? {};
  }

  const tlsOptions = options?.tlsOptions ?? {};
  tlsOptions.ca = ledgerIdentityCertificate;
  const confidentialLedger = GeneratedConfidentialLedger(ledgerBaseUrl, credentials!, {
    ...options,
    tlsOptions,
  });
  return confidentialLedger;
}

we might need to consider to support it in the code gen side

[qiaozha]

update the customization document first and then considering put it in the codegen if there're more cases.