Because:
* Plan eligibility depends on a number of factors, including what plans the user is already subscribed to (in Stripe, Google IAP or Apple IAP) and available upgrade paths for those plans per product-specific config (in Stripe metadata currently but soon to live in Contentful).
* We want to break out eligibility checks and types into a separate library from the auth server's `CapabilityService.getPlanEligibility`, since it's not really capability-oriented.
This commit:
* Generates a new nx library in lib/payments/eligibility
Closes #FXA-7582
Bianca Danforth