Allow plugins to submit audit requests/responses via extended SystemView (#6777)

Move audit.LogInput to sdk/logical.  Allow the Data values in audited
logical.Request and Response to implement OptMarshaler, in which case
we delegate hashing/serializing responsibility to them.  Add new
ClientCertificateSerialNumber audit request field.

SystemView can now be cast to ExtendedSystemView to expose the Auditor
interface, which allows submitting requests and responses to the audit
broker.

sdk/logical/system_view.go

@@ -70,6 +70,10 @@ type SystemView interface {
 	PluginEnv(context.Context) (*PluginEnvironment, error)
 }
 
+type ExtendedSystemView interface {
+	Auditor() Auditor
+}
+
 type StaticSystemView struct {
 	DefaultLeaseTTLVal  time.Duration
 	MaxLeaseTTLVal      time.Duration
@@ -86,6 +90,20 @@ type StaticSystemView struct {
 	PluginEnvironment   *PluginEnvironment
 }
 
+type noopAuditor struct{}
+
+func (a noopAuditor) AuditRequest(ctx context.Context, input *LogInput) error {
+	return nil
+}
+
+func (a noopAuditor) AuditResponse(ctx context.Context, input *LogInput) error {
+	return nil
+}
+
+func (d StaticSystemView) Auditor() Auditor {
+	return noopAuditor{}
+}
+
 func (d StaticSystemView) DefaultLeaseTTL() time.Duration {
 	return d.DefaultLeaseTTLVal
 }