Add Discount Enum Item

[armando-rodriguez-cko]

Description:

This PR introduces a new item type, DISCOUNT, in the PaymentContextsItemsType enumeration to enhance payment context flexibility.

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Key Changes:

PaymentContextsItemsType.java:

• Added a new enum item:
  • DISCOUNT
    • Annotated with @SerializedName("discount") to support accurate serialization and integration.

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Impact:

• Improved Payment Context Handling: Enables representation of discounts directly within payment contexts, broadening the range of supported payment item types.
• Seamless Integration: The @SerializedName ensures compatibility with existing APIs and serialization logic.

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Additional Notes:

• Backward compatibility is maintained, ensuring no disruption to existing workflows.
• This change aligns with evolving needs for more granular payment item types in the Checkout.com SDK.