Add hclog adapter and gating to eventer (#1915)

This contains to related things:

1. An adapter allowing an hclog interface to take in information and send them to the eventer
2. A gate on the eventer, which is the analogue to the behavior prior to eventing where we'd ensure logs were not printed prior to the startup information. Without this, plugin logs using the hclog adapter will start showing up too early.

internal/cmd/commands/server/server.go

@@ -164,7 +164,8 @@ func (c *Command) Run(args []string) int {
 	if err := c.SetupEventing(c.Logger,
 		c.StderrLock,
 		strings.Join(serverNames, "/"),
-		base.WithEventerConfig(c.Config.Eventing)); err != nil {
+		base.WithEventerConfig(c.Config.Eventing),
+		base.WithEventGating(true)); err != nil {
 		c.UI.Error(err.Error())
 		return base.CommandUserError
 	}
@@ -447,7 +448,10 @@ func (c *Command) Run(args []string) int {
 	}()
 
 	c.PrintInfo(c.UI)
-	c.ReleaseLogGate()
+	if err := c.ReleaseLogGate(); err != nil {
+		c.UI.Error(fmt.Errorf("Error releasing event gate: %w", err).Error())
+		return base.CommandCliError
+	}
 
 	if c.Config.Controller != nil {
 		c.EnabledPlugins = append(c.EnabledPlugins, base.EnabledPluginHostAws, base.EnabledPluginHostAzure)