[PAY-3648] Migrate buy-usdc to sdk

[dharit-tan]

Description

Tried to convert to sdk functions just based on types.

Not sure about error-handling scheme for solanaRelay? Old world uses {res, error, errorCode} and most of our fns expect that.

How Has This Been Tested?

Finding it difficult to test these flows in Thailand even with VPN

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Latest commit: d07caca

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