Make `snap_getCurrencyRate` support `SOL`

[GuillaumeRx]

Right now this call only supports BTC, make it so you can get conversion rate for other tokens on solana

[GuillaumeRx]

The plan is to deprecate the usage of snap_getCurrencyRate in favour of the snap handling the rates fetching logic and provide it to the extension. see MetaMask/SIPs#154