feat: allow users to set a custom endpoint

[ju5t]

You can deploy with Docker on your own platform. This means you need to be able to override the endpoint on which you will access its API.

@owenvoke let me know if this fits your style or if you want to call it FRANKFURTER_BASE_URL as that's the name of the variable in the provider.

[owenvoke]

Hi @ju5t, please could you name it FRANKFURTER_BASE_URL (and exchange.services.frankfurter.base_url for the config). 👍🏻 Otherwise, this looks good.

Didn't know you can self-host Frankfurter, interesting!

[ju5t]

@owenvoke it's now FRANKFURTER_BASE_URL. I changed the test and config names too of course.

[owenvoke]

Thanks!