feat(ContractNegotiation): add checksum (spec)

spi/control-plane/contract-spi/src/main/java/org/eclipse/edc/connector/contract/spi/types/negotiation/ContractNegotiation.java

@@ -66,6 +66,7 @@ public class ContractNegotiation extends StatefulEntity<ContractNegotiation> {
     private String correlationId;
     private String counterPartyId;
     private String counterPartyAddress;
+    private String checksum;
     private String protocol;
     private Type type = CONSUMER;
     private ContractAgreement contractAgreement;
@@ -83,6 +84,10 @@ public String getCounterPartyAddress() {
         return counterPartyAddress;
     }
 
+    public String getChecksum() {
+        return checksum;
+    }
+
     /**
      * Returns the correlation id sent by the client or null if this is a client-side negotiation.
      */
@@ -416,6 +421,11 @@ public Builder counterPartyAddress(String address) {
             return this;
         }
 
+        public Builder checksum(String checksum) {
+            entity.checksum = checksum;
+            return this;
+        }
+
         public Builder correlationId(String id) {
             entity.correlationId = id;
             return this;