Port over some SP v2 bits (#6516)

* Port over some SP v2 bits

Specifically:

* Add too-large handling to Physical (Consul only for now)
* Contextify some identity funcs
* Update SP protos

* Add size limiting to inmem storage

sdk/physical/physical.go

@@ -20,6 +20,10 @@ const (
 	PutOperation              = "put"
 )
 
+const (
+	ErrValueTooLarge = "put failed due to value being too large"
+)
+
 // ShutdownSignal
 type ShutdownChannel chan struct{}