feat(investment): add issueDate, purchaseDate, issuerCNPJ fields (#82)

The Pluggy API returns three Investment fields the DTO does not map. Under
Gson's FieldNamingPolicy.IDENTITY, Java field names must match the API JSON
keys byte-for-byte, so unmapped/misnamed fields are silently dropped to null.

- issueDate: the existing `issuerDate` field is a misnaming (extra `r`) with
  no matching API key, so it was always null. Added the correct `issueDate`
  and marked `issuerDate` @deprecated (kept for backward compatibility).
- purchaseDate: was not mapped.
- issuerCNPJ: was not mapped (casing verified against api.pluggy.ai/oas3.json,
  CNPJ is upper-case in the spec).

issueDate/purchaseDate are date-time strings -> Date, matching the existing
`date`/`dueDate` mapping; issuerCNPJ is a string.

Author: Gabrielpanga

src/main/java/ai/pluggy/client/response/Investment.java

@@ -36,7 +36,16 @@ public class Investment {
   String amountProfit = null;
   Double amountWithdrawal;
   String issuer;
+  String issuerCNPJ;
+  /**
+   * @deprecated the API field is {@code issueDate}; this misnamed field always
+   *             resolves to {@code null} under Gson's {@code IDENTITY} naming.
+   *             Use {@link #issueDate} instead.
+   */
+  @Deprecated
   Date issuerDate;
+  Date issueDate;
+  Date purchaseDate;
   InvestmentStatus status;
   /**
    * @deprecated use