Tax-Free Childcare: Add qualifying childcare provider checks

[nikhilwoodruff]

The current implementation correctly caps the tax-free childcare contribution by the actual childcare expenses in tax_free_childcare_benefits_amount.py, but there's no check that these expenses are paid to qualifying/registered childcare providers.

According to the legislation, only payments to registered and approved childcare providers qualify for the scheme.

Suggested improvement:

• Create a new variable to track whether childcare expenses are paid to qualifying providers
• Add this as a condition in the tax-free childcare eligibility check
• Consider adding parameters to define qualifying provider types

References:

• Childcare Payments Act 2014, Sections 47-51: https://www.legislation.gov.uk/ukpga/2014/28/part/2/chapter/5
• Section 47 (Qualifying childcare): https://www.legislation.gov.uk/ukpga/2014/28/section/47
• Section 48 (Registered or equivalent childcare): https://www.legislation.gov.uk/ukpga/2014/28/section/48