Support flow_data parameter in createPortalLink function

[chetbox]

Resolves #551

Example usage to create a portal link that asks the customer to confirm an amendment to the price and/or quanity of a subscription:

import { getFunctions, httpsCallable } from 'firebase/functions';

const flow_data = {
  type: 'subscription_update_confirm',
  subscription_update_confirm: {
    subscription: 'sub_XXXXXX',
    items: [
      { id: 'si_XXXXXX', quantity: 4, price: 'price_XXXXXX' },
    ],
  },
};

const portalLinkUrl = (await httpsCallable(getFunctions(app), 'createPortalLink')({ flow_data })).data.url;

[CLAassistant]

All committers have signed the CLA.

[chetbox]

@dackers86 does this now need to be merged into the next branch? That seems to be default branch but master was the default branch when this PR was created.

[dackers86]

HI @chetbox

Yes, you are correct! The base branch was updated today to be the next branch

[dackers86]

Moved to next 9ab8170 Thanks