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.

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[jar-stripe]

@hootanht Thank you submitting this PR! Supporting System.Text.Json as an alternative to Newtonsoft is on our to-do list for this quarter, and I'd love to work with you on getting this landed. We have a code generator that is responsible for generating some 500-odd files that I definitely don't want to ask you to manually update, and I want to be able to include this in such a way that one SDK will be compatible with users of both Newtonsoft and System.Text.Json. Let me review your changes and see how we can incorporate them into our code generation infrastructure, make sure it won't break existing users, and then I'll plan to push updates right back to this branch/PR. Sound good?

[hootanht]

@hootanht Thank you submitting this PR! Supporting System.Text.Json as an alternative to Newtonsoft is on our to-do list for this quarter, and I'd love to work with you on getting this landed. We have a code generator that is responsible for generating some 500-odd files that I definitely don't want to ask you to manually update, and I want to be able to include this in such a way that one SDK will be compatible with users of both Newtonsoft and System.Text.Json. Let me review your changes and see how we can incorporate them into our code generation infrastructure, make sure it won't break existing users, and then I'll plan to push updates right back to this branch/PR. Sound good?

Awesome 👌