[FlightSQL][C++] Expose is_update field of PreparedStatement

### Describe the enhancement requested

PR #49498 added `is_update` as field to the `ActionCreatePreparedStatementResult` protobuf message in `format/FlightSql.proto`. However, the C++ high-level wrapper layer has not been updated to expose this field.

This enhancement is aimed at allowing C++ Flight SQL server implementors to set `is_update` when creating a prepared statement, and C++ clients to read it. It does not contemplate the ODBC driver (see #51115 for that).

This is the C++ analogue of https://github.com/apache/arrow-go/issues/705

### Component(s)

FlightRPC, C++

Related
- #49497
- #49498
- ODBC driver follow-up: #51115

Start with format/FlightSql.proto and inspect the C++ high-level Flight SQL prepared-statement wrapper, using PR #49498 for the existing is_update field and its intended semantics. Expose the field for server creation and client access, and verify that both sides can set and read it without changing the ODBC driver.

Written by the indexing model from the issue text.