Implement other APIs

Problem/Motivation

It's a placeholder ticket (that will probably stay dead) for adding API integrations other than Fixer. No work is planned here unless there is a demand for that.

Proposed resolution

Redesign the code to be extendable, enabling the addition of new currency rate providers. Update the configuration schema accordingly and incorporate new providers.

Remaining tasks

• Get feedback.

User interface changes

API changes

Data model changes