Expose *sql.DB instead of *pgxpool.Pool as the public interface.
Uses pgx/v5/stdlib as the underlying driver. This enables compatibility
with standard Go database tooling (ORMs, query builders, migration
frameworks) and the Container Registry's load balancing code.

Address AGENTS.md review findings:
- Add Name field to Config with default "postgres" for multi-database
  identification in logs and errors
- Add ConnMaxLifetime and ConnMaxIdleTime config for PgBouncer compat
- Wrap Shutdown errors with the component name
- Use Start error prefix from c.name instead of hardcoded "postgres"
- Extract OTel attribute keys to package-level constants

- Rename MinConns to MaxIdleConns to match the database/sql API it
  maps to, avoiding confusion about the field's semantics
- Add db.name span attribute per OpenTelemetry database semantic
  conventions, improving trace filtering for multi-database services
- Add Name() tests for default and custom values

feat(v2/postgres): Add instrumented PostgreSQL client with query tracing

See merge request !340



Merged-by: Elliot Forbes <eforbes@gitlab.com>
Approved-by: Elliot Forbes <eforbes@gitlab.com>
Co-authored-by: dbarrett <dbarrett@gitlab.com>