feat: AssignMessage hygiene checks

checks for presence of elements, proper placement, etc.

README.md

@@ -276,6 +276,7 @@ This is the current list:
 |   |:white_check_mark:| PO031 | AssignMessage content-type | When assigning to Payload, you should also assign content-type, exactly once. |
 |   |:white_check_mark:| PO032 | CORS policy hygiene | In a CORS policy, wildcard origins should generate a warning. And other hygiene checks.|
 |   |:white_check_mark:| PO033 | ExtractVariables policy hygiene | In an ExtractVariables policy, check variable types and other hygiene. |
+|   |:white_check_mark:| PO034 | AssignMessage policy hygiene | In an AssignMessage policy, check element placement and other hygiene. |
 | FaultRules |   |   |   |   |
 |   |:white_check_mark:| FR001 | No Condition on FaultRule | Use Condition elements on FaultRules, unless it is the fallback rule. |
 |   |:white_check_mark:| FR002 | DefaultFaultRule Structure | DefaultFaultRule should have only supported child elements, at most one AlwaysEnforce element, and at most one Condition element. |