Add runtime support for V2 int64 string-encoded fields

[mbroshi-stripe]

Why?

V2 APIs represent int64 fields as JSON strings on the wire (e.g. "12345" instead of 12345) to avoid precision loss. The SDK should transparently convert between native Python int and this wire format so users don't need to handle string encoding themselves.

What?

• Added _coerce_v2_params() and _coerce_value() to stripe/_encode.py for request-side coercion (int → str), handling scalars, nested objects, and arrays
• Added _field_encodings class variable and _coerce_field_value() method to StripeObject for response-side coercion (str → int) during _refresh_from
• Added tests covering top-level, nested, array, nullable, and mixed-field scenarios for both request and response paths