adding portable did to keychain

[nitro-neal]

As we create the identity manager, we build it with a portable did, and save this portable did to keychain

  const portableDid: PortableDid = await DidIonMethod.create(didOptions);
  await setItemAsync(name, JSON.stringify(portableDid));

  const identity = await agent.identityManager.create({
    did: portableDid,
    name,
    kms
  });

the reason we do this is so we can access it later for creating vcs and signing