Add parameterized execute for postgres

[mcraveiro]

Summary

Add support for parameterized queries using PostgreSQL's $1, $2, ... placeholder syntax. This enables safe parameter binding for:

• Calling PostgreSQL functions without defining custom types
• Executing dynamic queries with SQL injection protection
• Passing NULL values via std::optional or std::nullopt

API

// Variadic execute with automatic type conversion
conn->execute("SELECT my_func($1, $2)", tenant_id, user_email);

// Supported types: string, numeric, bool, optional, nullptr
conn->execute("INSERT INTO t (a, b, c) VALUES ($1, $2, $3)",
              std::string("text"), 42, std::nullopt);

Implementation

• Add PostgresV2Result::make() overload using PQexecParams
• Add variadic execute() template to Connection with to_param() helper
• Internal execute_params() method handles the actual libpq call

Test plan

• execute_with_string_params - string and int parameters
• execute_with_null_param - NULL via std::optional
• execute_with_numeric_params - int, double, bool
• execute_call_function - calling PL/pgSQL function

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