Use configuration flag to log custom step event for GetStartedViewController

[selanthiraiyan]

Closes #723

Description

Previously in #674 we introduced a new step value enter_email_address for GetStartedViewController .
This was added to differentiate between GetStartedViewController and SiteAddressViewController in Woo mobile app. Those two screens had the same flow and step value.

This new step value change has led to an issue with the WPiOS Looker dashboard (ref: pe5sF9-FU-p2#comment-1542).
To fix the issue, we are introducing a configuration flag in WordPressAuthenticatorConfiguration to use custom step event only when the useEnterEmailAddressAsStepValueForGetStartedVC flag is enabled.

Testing steps

Follow the testing instructions from

1. WPAuthenticator - Config flag to enable custom step value for get started screen.  woocommerce/woocommerce-ios#8669
2. WPAuthenticator - Remove custom step value from GetStartedViewController WordPress-iOS#19940

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• I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[itsmeichigo]

LGTM 🚀