Setting up defailt exchange provider

[TChukwuleta]

Resolves #5653, allowing admin to setup default exchange provider

[NicolasDorier]

It will need a few changes and go after #5976, but I can take it from here.

I think we will need to talk to @dstrukt. This UX is simple, but lacking flexibility as it only allows to change the recommended exchange for the default currency.

On one side, maybe we don't really care about the lack of flexibility. An admin changing this setting will probably change it once in his life, as all his merchants should use the same default currency.

On the other side, it doesn't allow an admin to change the recommended exchanges for other currencies.

[TChukwuleta]

I think the flexibility would be good one to have.

I have a follow up question, if an admin has merchants in different countries (say Nigeria and Ghana), how would the currencies be managed if the intentions is for all the merchants to use the same default currency

[NicolasDorier]

@pavlenex @dstrukt we will need some guidance here

So from #5976, it is possible to take whatever exchange is recommended for the selected currency. (rather than forcing the same exchange for all currencies)

This PR aims at allowing the admin to setup the recommended exchange for the default currency.

The good side is that this is simple to understand.

The bad side is that it doesn't allow the customization of recommended exchange for non default currency.

I am tempted to keep it simple like this PR, as our original goal is to allow local admin to streamline the experience for their local merchant. (ie, for the default currency)

Note this PR may need some adjustement to work, but we need to know the path to take.

[dstrukt]

Discussed on today's design call. Not sure if @dennisreimann is going to implement the additional dropdown solution we discussed, so will leave it to him add any additional context, or if @dennisreimann is planning to leave it to @TChukwuleta, regardless, thought I'd provide an update.

[TChukwuleta]

Thanks @dstrukt

[dennisreimann]

tACK. Ideally we'd also combine this with the rate provider recommendations, e.g. Kraken for EUR — but I think this is more complex and would involve some more JS.