Document snap_getCurrencyRate

[Montoya]

This is a new method that is not documented. The method is here:

https://github.com/MetaMask/snaps/blob/ec811b7d140fd6a4ce3e7e68970bef7c55e89162/packages/snaps-sdk/src/types/methods/get-currency-rate.ts#L23

It allows a Snap to get the internal currency conversion from MetaMask

[ajeetd]

Get the exchange value for the requested crypto against the users preferried fiat currency in the extension. Currently only BTC.

[ziad-saab]

@Montoya or @ajeetd : I tried the latest published version of the snaps-sdk and it doesn't yet have that method in its types. i think i should be able to get around that for testing, if I can find an appropriate version of Flask to test on. can you please provide one?