Implement `Adapter` interface for network policy enforcement

## Context

The core `xk6-sql` extension is introducing an `Adapter` interface to support centralized network validation (grafana/xk6-sql#177). This change utilizes the `GetAddrResolver()` functionality introduced in k6 v1.5.0 to enforce network policies (such as `blockHostnames`, `hosts` file mappings, and DNS rules) before establishing a database connection.

Enforcing these policies is a requirement for `xk6-sql` and its drivers to comply with k6 extension standards.

## Goal
Update the Postgres driver to implement the new `Adapter` interface. This allows the driver to expose the target hostname(s) to the `xk6-sql` core for validation without requiring the core to contain Postgres-specific parsing logic.

## Interface Definition (from xk6-sql)

```go
package sql

// DataSourceInfo holds properties extracted from a Data Source Name (DSN).
type DataSourceInfo struct {
// Addrs is a list of "host" or "host:port" strings found in the DSN.
Addrs []string
}

// Adapter defines the interface that driver extensions can optionally implement
// to provide parsing logic.
type Adapter interface {
ParseDSN(dsn string) (DataSourceInfo, error)
}

// RegisterAdapter registers a driver with a custom adapter for DSN parsing.
// This allows the driver to opt-in to core validation logic.
//
// Note: This function registers the driver name internally.
// Extensions using RegisterAdapter do NOT need to call RegisterDriver separately.
func RegisterAdapter(driverName string, adapter Adapter) *sobek.Symbol
```

## Task
1. **Implement the Adapter:** Create a struct that satisfies the `sql.Adapter` interface.
2. **Parse DSN:** Implement the `ParseDSN` method to extract the target address (host and port) from the connection string.
* *Note:* Postgres supports both Keyword/Value connection strings (e.g., `host=localhost port=5432`) and Connection URIs (e.g., `postgres://user:pass@localhost:5432/db`). The parser must handle both formats to correctly identify the host.
3. **Update Registration:** Change the driver registration to use `sql.RegisterAdapter` (or the equivalent module-level wrapper if applicable). Note that `RegisterAdapter` handles the driver name registration, so the previous registration call should be replaced.