allow admin update default rate provider and default currency for stores

[TChukwuleta]

resolve #5653

Allow server admin can change currencies and providers for stores.

Cc. @dennisreimann

[TChukwuleta]

this is up for review :)

[NicolasDorier]

@TChukwuleta the idea isn't to give the admin the ability to change the config of individual stores.

The idea is to give the admin the ability to change what is selected by default in the "Create Store" wizard, so that new users don't have to select their currency and the right exchange manually.

It makes it possible for local admin to optimize the UX specifically for their local community.

[TChukwuleta]

Oh. I see, thank you @NicolasDorier

[TChukwuleta]

Hey @NicolasDorier I updated the policy section in server settings to give the admin the ability to update default currency, which is now used as default for create currency, and in scenarios where there is none, it selects the previous default USD (storeblob.defaultcurrency)

[NicolasDorier]

Did a minor rewording of the label.