Add configuration to disable site credential login on Get Started screen

[itsmeichigo]

Part of https://github.com/woocommerce/woocommerce-ios/issues/8845.

Description This PR adds a new config enableSiteCredentialLoginForJetpackSites - which is true by default. When this is set to false, the site credential login button will not be displayed on the WPCom email login screen of the site address login flow.

Testing steps Please follow the steps in https://github.com/woocommerce/woocommerce-ios/pull/8846.

Screenshot

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