Implement situational enablement of split UPE feature flag to facilitate rollout

[FangedParakeet]

Description

Please read this thread for more details on this issue.

We are planning to roll out the split UPE project gradually behind a feature flag that is under development in #5353 (WIP PR in #5496). We would like to implement some custom rules, so that users who are new to the UPE or WCPay have this feature enabled automatically, whereas existing customers using the current UPE implementation are not automatically and unwillingly migrated to the split UPE feature.

Acceptance criteria

We seek to implement the following rules.

• New stores that register with WCPay automatically have the split UPE flag enabled.
• Existing stores that are already registered with WCPay, but do not currently have any version of the UPE enabled via the current feature flag, will enable the split UPE flag when they decide to enable the "new checkout experience" in their WCPay settings.
• Existing stores that do have some version of the UPE enabled via the current feature flag will not automatically find that the new split UPE flag is enabled.
• Existing stores that do have some version of the UPE enabled via the current feature flag can enable the split UPE feature flag by disabling the "new checkout experience" and then re-enabling it.

Additional context

p1674500132021179/1674491221.244469-slack-C048UEB37MZ
#5353
#5496

[FangedParakeet]

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