Support inline SSL certificates

pq just added this feature https://github.com/lib/pq/pull/818/files and it would be really nice to have this here because in the containers world certs are usually passed to apps as environment variables.

Now we have to manually compose TLS config:

```go
pair, err := tls.X509KeyPair(cert, key)
if err != nil {
return err
}
config := &tls.Config{
ServerName: serverName,
Certificates: []tls.Certificate{pair},
RootCAs: x509.NewCertPool(),
}
if !config.RootCAs.AppendCertsFromPEM(ca) {
return errors.New("cannot append root CA certificates")
}
poolConfig.ConnConfig.TLSConfig = config
```

Start by reading the linked lib/pq pull request and then locate pgx’s existing TLS configuration entry point. Compare the requested inline certificate, key, and CA handling with the current manual tls.Config example. Done means callers can provide certificate values in the container-oriented form described without assembling that TLS configuration themselves.

Written by the indexing model from the issue text.