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 long and this wire format so users don't need to handle string encoding themselves.

What?

• Added StringInt64Converter (Newtonsoft.Json) — reads string or integer tokens, writes as string
• Added STJStringInt64Converter (System.Text.Json, .NET 6+) — same behavior for STJ
• Added tests for deserialization from string, deserialization from integer, and serialization to string
• Generated code will annotate int64_string fields with [JsonConverter(typeof(StringInt64Converter))]