Plaid Check income verification eligibility and optimize conversion

`/credit/payroll_income/precheck` is an optional endpoint that can be called before initializing a Link session for income verification. It evaluates whether a given user is supportable by digital income verification. If the user is eligible for digital verification, that information will be associated with the user token, and in this way will generate a Link UI optimized for the end user and their specific employer. If the user cannot be confirmed as eligible, the user can still use the inco...