[Feature] Incorporating a variable and for loop for churn reason

### Is there an existing feature request for this?

- [X] I have searched the existing issues

### Describe the Feature

Churn reasons are currently mostly customized in our `recurly__churn_analysis` based on internal logic in the Recurly Churn API. https://docs.recurly.com/docs/churn-analysis

However, it's likely customers have their own internal churn logic that differs from Recurly. It makes sense for us to incorporate additional variables so you can customize exactly what churn looks like based on internal definitions, then implementing a for loop to complete the data.

### Describe alternatives you've considered

One possible way to make the variable work is to leverage a dictionary, then use a key value pair for loop that would be able to provide you all the necessary logic to produce your own churn reasons!

Here's an example of a key value pair for loop in action: https://github.com/fivetran/dbt_salesforce_formula_utils/blob/main/macros/sfdc_formula_refactor.sql

### Are you interested in contributing this feature?

- [X] Yes.
- [ ] Yes, but I will need assistance and will schedule time during your [office hours](https://calendly.com/fivetran-solutions-team/fivetran-solutions-team-office-hours) for guidance.
- [ ] No.

### Anything else?

_No response_