create subscription to have only plan_id as the required feature

We need to allow setting default values at a plan level for below fields:
- billing_cadence
- billing_period
- currency
Ref - https://docs.flexprice.io/api-reference/subscriptions/create-subscription

Also need to make sure that if defaults are set, we do not allow adding prices of any other currency/billing_period or cadence

This will then make subscription creating very simple with only two params - plan_id and customer_id

We didn't do this in the first place because flexprice allows to set up create a plan with monthly and annual both prices but we have realised that it is not the best way forward given how entitlements and credit grants also change within a plan so we want to steer towards restricting these at a plan level and make the overall experience better.