* Support pluginable rate providers
This PR allows plugins to provide custom rate providers, that can be contextual to a store. For example, if you use the upcoming fiat offramp plugin, or the Blink plugin, you'll probably want to configure the fetch the rates from them since they are determining the actual fiat rrate to you. However, they require API keys. This PR enables these scenarios, even much more advanced ones, but for example:
* Install fiat offramp plugin
* Configure it
* You can now use the fiat offramp rate provider (no additional config steps beyond selecting the rate source from the select, or maybe the plugin would automatically set it for you once configured)
* Apply suggestions from code review
* Simplify
* Do not use BackgroundFetcherRateProvider for contextual rate prov
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Co-authored-by: nicolas.dorier <nicolas.dorier@gmail.com>
