Support currencies that are not in the Frankfurter API

[Petersmit27]

Currently, Spliit supports currency conversion between currencies in the frankfurter API (https://api.frankfurter.dev/v1/currencies). This list is unfortunately not exhaustive of all currencies, and there are people asking for broader currency support (#418, #431, #438).

The obvious solution is to find a better API, but I have been unable to one that is free-to-use with similar conversion rate functionality. If you do know of one, please tell us.

I'm creating this issue to discuss (and eventually implement) an approach that would easily scale to more currencies and is as user-friendly as possible.

I think @derekl-beep's idea of supporting the currency, but the user needs to set a conversion rate themselves (as discussed in #431 is good, but there are some things I think that should be improved:

• Let the user know this currency does not have conversion rate support
  • In the group settings, show a hint text that says that the user should fill in conversion rates themselves
  • Don't try to fetch the conversion rate from Frankfurter if it's not supported anyway
• Store conversion rate locally (if it isn't already)
• Warn the user if the conversion rate was set more than x days ago, since the conversion rate has probably changed significantly since then

Let me know if you have more ideas