Mollie API 4.5.0.0

New features:

• Add Google Pay to the list of payments methods: #408
• Add support for vouchers in the Payments API as described in the Mollie docs on integrating vouchers.

• Add Voucher to the list of payments methods
• Added Categories property to the PaymentLine class.
• Added Issuer property to PaymentRequest class.
• Moved OrderLineDetailsCategory to VoucherCategory, as it can now be used by both the Order and Payment API.

Technical improvements:

• Make currency codes constant strings instead of static strings, so they can no longer be changed: #403
• Replace FluentAssertions with Shouldly, due to FluentAssertions changing their licensing model for future versions.
• Reduce flakyness of integration tests using a Polly retry mechanism on 429 errors.