Support JSON serialization with System.Text.Json

[hootanht]

Fixes #2495

Add support for JSON serialization with System.Text.Json.

• Add JsonSerializer interface and implement NewtonsoftJsonSerializer and SystemTextJsonSerializer classes in src/Stripe.net/Infrastructure/JsonSerializer.cs.
• Update StripeEntity class in src/Stripe.net/Entities/_base/StripeEntity.cs to use the JsonSerializer interface for ToJson and FromJson methods.
• Replace Newtonsoft.Json attributes with System.Text.Json attributes in src/Stripe.net/Entities/Customers/Customer.cs, src/Stripe.net/Entities/_common/Address.cs, src/Stripe.net/Entities/_common/AddressJapan.cs, src/Stripe.net/Entities/_common/Shipping.cs, src/Stripe.net/Entities/AccountLinks/AccountLink.cs, and src/Stripe.net/Entities/Accounts/Account.cs.
• Add conditional compilation to use appropriate attributes based on the selected JSON serializer in the above files.

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