Add connection option WithSkipTLSHostVerify for privatelink host (#225)

## Background
The driver cannot connect to the workspace whose hostname is an internal
private link hostname, this is because its domain is not added into
Databricks workspace certificate. See
#223

## Description
This change adds connection option `WithSkipTLSHostVerify` which can
disable verifying the hostname in the server certificate. In this mode,
TLS is susceptible to machine-in-the-middle attacks. Please only use
this option when the hostname is an internal private link hostname.

Here is the usage
```go
connector, err := dbsql.NewConnector(
   dbsql.WithServerHostname("<hostname>"),
   dbsql.WithHTTPPath("<http_path>"),
   dbsql.WithAccessToken("<token>"),
   dbsql.WithSkipTLSHostVerify(),
)
```

connector.go

@@ -2,6 +2,7 @@ package dbsql
 
 import (
 	"context"
+	"crypto/tls"
 	"database/sql/driver"
 	"fmt"
 	"net/http"
@@ -233,6 +234,20 @@ func WithSessionParams(params map[string]string) connOption {
 	}
 }
 
+// WithSkipTLSHostVerify disables the verification of the hostname in the TLS certificate.
+// WARNING:
+// When this option is used, TLS is susceptible to machine-in-the-middle attacks.
+// Please only use this option when the hostname is an internal private link hostname
+func WithSkipTLSHostVerify() connOption {
+	return func(c *config.Config) {
+		if c.TLSConfig == nil {
+			c.TLSConfig = &tls.Config{MinVersion: tls.VersionTLS12, InsecureSkipVerify: true} // #nosec G402
+		} else {
+			c.TLSConfig.InsecureSkipVerify = true // #nosec G402
+		}
+	}
+}
+
 // WithAuthenticator sets up the Authentication. Mandatory if access token is not provided.
 func WithAuthenticator(authr auth.Authenticator) connOption {
 	return func(c *config.Config) {

connector_test.go

@@ -6,7 +6,9 @@ import (
 	"time"
 
 	"github.com/databricks/databricks-sql-go/auth/pat"
+	"github.com/databricks/databricks-sql-go/internal/client"
 	"github.com/databricks/databricks-sql-go/internal/config"
+	"github.com/hashicorp/go-retryablehttp"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -38,6 +40,7 @@ func TestNewConnector(t *testing.T) {
 			WithTransport(roundTripper),
 			WithCloudFetch(true),
 			WithMaxDownloadThreads(15),
+			WithSkipTLSHostVerify(),
 		)
 		expectedCloudFetchConfig := config.CloudFetchConfig{
 			UseCloudFetch:      true,
@@ -67,6 +70,7 @@ func TestNewConnector(t *testing.T) {
 		expectedCfg := config.WithDefaults()
 		expectedCfg.DriverVersion = DriverVersion
 		expectedCfg.UserConfig = expectedUserConfig
+		expectedCfg.TLSConfig.InsecureSkipVerify = true
 		coni, ok := con.(*connector)
 		require.True(t, ok)
 		assert.Nil(t, err)
@@ -184,6 +188,28 @@ func TestNewConnector(t *testing.T) {
 		}
 
 	})
+
+	t.Run("Connector test WithSkipTLSHostVerify with PoolClient", func(t *testing.T) {
+		hostname := "databricks-host"
+		con, err := NewConnector(
+			WithServerHostname(hostname),
+			WithSkipTLSHostVerify(),
+		)
+		assert.Nil(t, err)
+
+		coni, ok := con.(*connector)
+		require.True(t, ok)
+		userConfig := coni.cfg.UserConfig
+		require.Equal(t, hostname, userConfig.Host)
+
+		httpClient, ok := coni.client.Transport.(*retryablehttp.RoundTripper)
+		require.True(t, ok)
+		poolClient, ok := httpClient.Client.HTTPClient.Transport.(*client.Transport)
+		require.True(t, ok)
+		internalClient, ok := poolClient.Base.(*http.Transport)
+		require.True(t, ok)
+		require.True(t, internalClient.TLSClientConfig.InsecureSkipVerify)
+	})
 }
 
 type mockRoundTripper struct{}

internal/client/client.go

@@ -2,6 +2,7 @@ package client
 
 import (
 	"context"
+	"crypto/tls"
 	"crypto/x509"
 	"encoding/json"
 	"fmt"
@@ -545,14 +546,20 @@ func RetryableClient(cfg *config.Config) *http.Client {
 	return retryableClient.StandardClient()
 }
 
-func PooledTransport() *http.Transport {
+func PooledTransport(cfg *config.Config) *http.Transport {
+	var tlsConfig *tls.Config
+	if (cfg.TLSConfig != nil) && cfg.TLSConfig.InsecureSkipVerify {
+		tlsConfig = cfg.TLSConfig
+	}
+
 	transport := &http.Transport{
 		Proxy: http.ProxyFromEnvironment,
 		DialContext: (&net.Dialer{
 			Timeout:   30 * time.Second,
 			KeepAlive: 30 * time.Second,
 			DualStack: true,
 		}).DialContext,
+		TLSClientConfig:       tlsConfig,
 		ForceAttemptHTTP2:     true,
 		MaxIdleConns:          100,
 		IdleConnTimeout:       180 * time.Second,
@@ -577,7 +584,7 @@ func PooledClient(cfg *config.Config) *http.Client {
 		}
 	} else {
 		tr = &Transport{
-			Base:  PooledTransport(),
+			Base:  PooledTransport(cfg),
 			Authr: cfg.Authenticator,
 		}
 	}