events: Add websockets and command (#19057)

Also updates the event receieved to include a timestamp.
Websockets support both JSON and protobuf binary formats.

This can be used by either `wscat` or the new
`vault events subscribe`:

e.g.,
```sh
$ wscat -H "X-Vault-Token: $(vault print token)" --connect ws://127.0.0.1:8200/v1/sys/events/subscribe/abc?json=true
{"event":{"id":"5c5c8c83-bf43-7da5-fe88-fc3cac814b2e", "note":"testing"}, "eventType":"abc", "timestamp":"2023-02-07T18:40:50.598408Z"}
...
```

and

```sh
$ vault events subscribe abc
{"event":{"id":"5c5c8c83-bf43-7da5-fe88-fc3cac814b2e", "note":"testing"}, "eventType":"abc", "timestamp":"2023-02-07T18:40:50.598408Z"}
...
```

Co-authored-by: Tom Proctor <tomhjp@users.noreply.github.com>

command/commands.go

@@ -332,6 +332,11 @@ func initCommands(ui, serverCmdUi cli.Ui, runOpts *RunOptions) map[string]cli.Co
 				BaseCommand: getBaseCommand(),
 			}, nil
 		},
+		"events subscribe": func() (cli.Command, error) {
+			return &EventsSubscribeCommands{
+				BaseCommand: getBaseCommand(),
+			}, nil
+		},
 		"lease": func() (cli.Command, error) {
 			return &LeaseCommand{
 				BaseCommand: getBaseCommand(),