demo: Get a temporary license upon demo startup

There is a wider discussion/design going on to serve licenses in
a more general way in the future, so this commit is not aiming
for a future-proof design, but instead an MVP to allow users to
demo enterprise features within `cockroach demo`.

We are not concerned about offline usage of enterprise features
as users can obtain a license and enable features manually using SET.

Fixes cockroachdb#40222.

Release note (cli change): cockroach demo attempts to contact a license
server to obtain a temporary license. cockroach demo now enables
telemetry for the demo cluster. This feature can be opted out of by
setting the `COCKROACH_SKIP_ENABLING_DIAGNOSTIC_REPORTING` environment variable
(https://www.cockroachlabs.com/docs/stable/diagnostics-reporting.html).

pkg/ccl/cliccl/demo.go

@@ -46,7 +46,6 @@ func getLicense(clusterID uuid.UUID) (string, error) {
 		return "", err
 	}
 	defer resp.Body.Close()
-
 	if resp.StatusCode != http.StatusOK {
 		return "", errors.New("unable to connect to licensing endpoint")
 	}

pkg/cli/demo.go

@@ -49,9 +49,9 @@ subcommands: e.g. "cockroach demo startrek". See --help for a full list.
 By default, the 'movr' dataset is pre-loaded. You can also use --empty
 to avoid pre-loading a dataset.
 
-cockroach demo attempts to connect to a Cockroach Labs server to obtain a
-temporary enterprise license for demoing enterprise features and enable
-telemetry back to Cockroach Labs. In order to disable this behavior, set the
+cockroach demo attempts to connect to a Cockroach Labs server to obtain a 
+temporary enterprise license for demoing enterprise features and enable 
+telemetry back to Cockroach Labs. In order to disable this behavior, set the 
 environment variable "COCKROACH_SKIP_ENABLING_DIAGNOSTIC_REPORTING".
 `,
 	Example: `  cockroach demo`,