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/audit.go

@@ -3,8 +3,8 @@ package logical
 type LogInput struct {
 	Type                string
 	Auth                *Auth
-	Request             interface{}
-	Response            interface{}
+	Request             *Request
+	Response            *Response
 	OuterErr            error
 	NonHMACReqDataKeys  []string
 	NonHMACRespDataKeys []string