GH-40097: [Go][FlightRPC] Enable disabling TLS (#40098)

See #40097 for more in-depth description
about the problem that led me to file this PR.

### Rationale for this change

Because it's annoying to not be able to connect to a non-TLS flightsql endpoint
in my development environment just because my development environment happens
to still use token authentication.

### What changes are included in this PR?

Thread the flightsql `DriverConfig.TLSEnabled` parameter into the
`grpcCredentials` type so that `grpcCredentials.RequireTransportSecurity` can
return false if TLS is not enabled on the driver config.

One thing that occurred to me about the `DriverConfig.TLSEnabled` field is that
its semantics seem very mildly dangerous since golang `bool` types are `false`
by default and golang doesn't require fields on structs to be explicitly
initialized. It seems to me that `DriverConfig.TLSDisabled` would be better (semantically speaking)
because then the API user doesn't have to explicitly enable TLS. But I suppose
it's probably undesirable to change the name of a public field on a public type.

### Are these changes tested?

I haven't written any tests, mostly because there weren't already any tests for
the `grpcCredentials` type but I have manually verified this fixes the problem
I described in #40097 by rebuilding my
tool and running it against the non-TLS listening thing in my development
environment.

### Are there any user-facing changes?

* Closes: #40097

Authored-by: wayne warren <wayne.warren.s@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>

go/arrow/flight/flightsql/driver/driver.go

@@ -364,10 +364,11 @@ func (c *Connector) Configure(config *DriverConfig) error {
 
 	// Set authentication credentials
 	rpcCreds := grpcCredentials{
-		username: config.Username,
-		password: config.Password,
-		token:    config.Token,
-		params:   config.Params,
+		username:   config.Username,
+		password:   config.Password,
+		token:      config.Token,
+		params:     config.Params,
+		tlsEnabled: config.TLSEnabled,
 	}
 	c.options = append(c.options, grpc.WithPerRPCCredentials(rpcCreds))
 

go/arrow/flight/flightsql/driver/utils.go

@@ -27,10 +27,11 @@ import (
 
 // *** GRPC helpers ***
 type grpcCredentials struct {
-	username string
-	password string
-	token    string
-	params   map[string]string
+	username   string
+	password   string
+	token      string
+	params     map[string]string
+	tlsEnabled bool
 }
 
 func (g grpcCredentials) GetRequestMetadata(ctx context.Context, uri ...string) (map[string]string, error) {
@@ -53,7 +54,7 @@ func (g grpcCredentials) GetRequestMetadata(ctx context.Context, uri ...string)
 }
 
 func (g grpcCredentials) RequireTransportSecurity() bool {
-	return g.token != "" || g.username != ""
+	return g.tlsEnabled && (g.token != "" || g.username != "")
 }
 
 // *** Type conversions ***