Add saveMultiple() method to FixedRates exchanger plugin

Problem/Motivation

Currently, the FixedRates exchanger plugin does not provide a method to save multiple exchange rates at once. This can lead to inefficient code when updating many rates, as each must be saved individually.

Steps to reproduce

Proposed resolution

Introduce a new method saveMultiple(array $newRates) in the FixedRates exchanger plugin. This method will allow batch updating of exchange rates by modifying the configuration in one operation.

Remaining tasks

User interface changes

API changes

Data model changes