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 StringInt64TypeAdapter (GSON TypeAdapter) — serializes Long as JSON string, deserializes string-encoded integers back to Long
• Added tests for request serialization (toMap produces string) and response deserialization (fromJson with string value produces Long)
• Generated code will annotate int64_string fields with @JsonAdapter(StringInt64TypeAdapter.class)