contracts: Allow indeterministic instructions off-chain (#12469)

* Allow indetermistic instructions off-chain

* Apply suggestions from code review

Co-authored-by: Sasha Gryaznov <hi@agryaznov.com>

* fmt

Co-authored-by: Sasha Gryaznov <hi@agryaznov.com>

substrate/frame/contracts/fixtures/delegate_call_simple.wat

@@ -0,0 +1,50 @@
+;; Just delegate call into the passed code hash and assert success.
+(module
+	(import "seal0" "seal_input" (func $seal_input (param i32 i32)))
+	(import "seal0" "seal_delegate_call" (func $seal_delegate_call (param i32 i32 i32 i32 i32 i32) (result i32)))
+	(import "env" "memory" (memory 3 3))
+
+	;; [0, 32) buffer where input is copied
+
+	;; [32, 36) size of the input buffer
+	(data (i32.const 32) "\20")
+
+	(func $assert (param i32)
+		(block $ok
+			(br_if $ok
+				(get_local 0)
+			)
+			(unreachable)
+		)
+	)
+
+	(func (export "call")
+		;; Reading "callee" code_hash
+		(call $seal_input (i32.const 0) (i32.const 32))
+
+		;; assert input size == 32
+		(call $assert
+			(i32.eq
+				(i32.load (i32.const 32))
+				(i32.const 32)
+			)
+		)
+
+		;; Delegate call into passed code hash
+		(call $assert
+			(i32.eq
+				(call $seal_delegate_call
+					(i32.const 0)	;; Set no call flags
+					(i32.const 0)	;; Pointer to "callee" code_hash.
+					(i32.const 0)	;; Input is ignored
+					(i32.const 0)	;; Length of the input
+					(i32.const 4294967295)	;; u32 max sentinel value: do not copy output
+					(i32.const 0)	;; Length is ignored in this case
+				)
+				(i32.const 0)
+			)
+		)
+	)
+
+	(func (export "deploy"))
+)