Add preferredProvider in buyWithFiat

[edwardysun]

Problem solved

Short description of the bug fixed or feature added

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PR-Codex overview

The focus of this PR is to add a preferredProvider option to the buyWithFiat functionality for selecting payment providers.

Detailed summary

• Added preferredProvider option to buyWithFiat functions
• Defined FiatProvider type with options: "STRIPE", "TRANSAK", "KADO"
• Updated multiple files to include preferredProvider option for fiat transactions

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