Allow host apps to insert custom UI instead of WP.com account creation flow

[ctarda]

Closes #533
Ref: woocommerce/woocommerce-ios#3146

Changes

Building up upon the functionality implemented in PR #525 , this PR touches GetStartedViewController, so that:

• When we detect an error after a user attempts log in with an email address, we would ask the host app if it wants to handle this error and intercept the navigation flow
• If the host app wants to handle the error, we would insert the UI that the host app provides.
• If the host app does not want to handle the error, we continue forward without changes in behaviour.
• Bumped the version in the podspec

How to test

This PR does not break the public API, but it has the potential to introduce a bug in WordPress. There are two ways I would suggest to test the functionality:

1. Test the corresponding PR in WooCommerce (Block sign ups to WordPress.com accounts. woocommerce/woocommerce-ios#3286). When users try to log in with a WordPress account and enter an email address that does not match a WP.com account, WooCommerce should insert custom UI.
2. Test the same flow with WordPress. No behaviour should change and it should still be possible to sign up for a WP.com account. The easiest way to test this is testing the draft PR Point WPAuthenticator to the branch that allows host apps to block account creation WordPress-iOS#15435

[mindgraffiti]

✅ Tested with WPiOS draft PR
✅ Code

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