Use more flexible interest rate type

[hieronx]

Which part of the code is the issue addressing?

• [ ] Addressing a bug
• [x] Refactoring
• [ ] Feature

Description

• [ ] Add new InterestRateType to cfg-types:

  pub enum InterestRate<Rate> {
  Fixed(Rate)
  }

• [x] Use new InterestRate enum for associated type of interest-accrual pallet: https://github.com/centrifuge/centrifuge-chain/blob/main/pallets/interest-accrual/src/lib.rs#L143 and adapt methods to accept this new data type
• [x] Adapt https://github.com/centrifuge/centrifuge-chain/blob/main/pallets/loans/src/types.rs#LL153C36-L153C40
• [x] Write migration for loans
• [ ] Adapt https://github.com/centrifuge/centrifuge-chain/blob/main/libs/types/src/tranches.rs#L96
• [ ] Write migration for tranches

[lemunozm]

@offerijns I don't fully understand this change. Why it's needed?