Loans: Fixed maturity with extension

[hieronx]

Description

Add an option to include a # of extension days in a fixed maturity asset, which enables extending the maturity by those # of days without going through the change guard.

The benefit is that for internally priced assets, the valuation can depend on the correct maturity date.

• Add

enum MaturityDate {
  Fixed {
    date: Moment,
    extension: Moment,
  }
}

• Maturity date can be modified as long as

  new_maturity_date = old_maturity_date + (prev_extension_days - remaining_extension_days)
  remaining_extension_days = prev_extension_days - (new_maturity_date - old_maturity_date)`

  Basically the extension days are reduced with the same amount as the maturity date is increased.

[lemunozm]

This issue is continued in this slack thread