feat: Add pay in advance logic into date service

[vincent-pochet]

Context

We want to add the ability to choose to bill users at subscription date anniversary and not only on a calendar basis.

Description

Following the previous introduction of a date service (see #360) to extract logic to compute fees boundaries, few changes are required in order to use this service in the Fees::ChargeService and Fees::SubscriptionService:

• from_date and to_date should return the boundaries of the coming subscription period when plan is payed in advance. It will kill the need for boundaries rewriting in Fees::SubscriptionService (see https://github.com/getlago/lago-api/blob/main/app/services/fees/subscription_service.rb#L152 for example)
• Since to_date is also used for charge, a charges_to_date is required as charges are always billed on the passed billing period no matter if the plan is payed in advance or in arrear
• a single_day_price method is required to compute the cost of a single subscription day based on the plan cost and the period duration
• a current_usage flag is required to return forecast dates for charges boundaries

[rsempe]

LGTM 🚀 - The QA part will be fun! 😄