Add duckarrow_execute() for DDL/DML statement execution

[JC1738]

Summary

• Add duckarrow_execute(sql) scalar function to execute DDL/DML statements on the remote Flight SQL server
• Addresses limitation where DROP TABLE duckarrow.X fails because DuckDB's replacement scan only intercepts table references in SELECT contexts
• Returns affected row count (BIGINT)

Changes

• execute_function.go - New scalar function implementation
• internal/flight/client.go - Add Execute() method using ADBC's ExecuteUpdate()
• main.go - Register new function during extension init
• README.md - Add usage documentation

Usage

-- Configure connection first
SELECT duckarrow_configure('grpc+tls://localhost:31337', 'user', 'pass');

-- Execute DDL/DML
SELECT duckarrow_execute('CREATE TABLE "test" (id INTEGER, name VARCHAR)');
SELECT duckarrow_execute('INSERT INTO "test" VALUES (1, ''Alice'')');
SELECT duckarrow_execute('DROP TABLE "test"');

Security

• SQL input validated for max length (1MB)
• Null bytes rejected to prevent truncation attacks
• Remote server errors prefixed for clarity

Test plan

• Build succeeds (make build)
• Unit tests pass (make test-unit)
• Integration test with Flight SQL server
• Verify DDL execution (CREATE/DROP TABLE)
• Verify DML execution (INSERT/UPDATE/DELETE)