Add preferredProvider option for thirdweb pay

[Yash094]

Adding the preferredProvider option for thirdweb pay.

Reference: https://portal.thirdweb.com/references/typescript/v5/GetBuyWithFiatQuoteParams

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PR-Codex overview

This PR introduces a new parameter preferredProvider to the BuyWithFiatQuoteParams class and updates its usage in the ThirdwebPay.GetBuyWithFiatQuote method. It also includes commented-out code in Program.cs to demonstrate how to use the new parameter.

Detailed summary

• Added preferredProvider parameter to BuyWithFiatQuoteParams class.
• Updated the ThirdwebPay.GetBuyWithFiatQuote method to include preferredProvider in the parameters.
• Included commented-out code in Program.cs to show usage of preferredProvider with an example.

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