Add the ability to distinguish map key violations (#55)

Example output from running new conformance tests against current
protovalidate-go:

![image](https://github.com/bufbuild/protovalidate/assets/4567082/431c9aa4-eefc-4ced-952f-ccd18a4bdc7b)

proto/protovalidate/buf/validate/expression.proto

@@ -83,4 +83,7 @@ message Violation {
   // `message` is a human-readable error message that describes the nature of the violation.
   // This can be the default error message from the violated `Constraint`, or it can be a custom message that gives more context about the violation.
   string message = 3;
+
+  // `for_key` indicates whether the violation was caused by a map key, rather than a value.
+  bool for_key = 4;
 }

tools/protovalidate-conformance/internal/cases/cases_map.go

@@ -138,6 +138,7 @@ func mapSuite() suites.Suite {
 					FieldPath:    "val[1]",
 					ConstraintId: "sint64.lt",
 					Message:      "value must be less than 0",
+					ForKey:       true,
 				},
 			),
 		},
@@ -148,6 +149,7 @@ func mapSuite() suites.Suite {
 					FieldPath:    "val[\"!@#$%^&*()\"]",
 					ConstraintId: "string.pattern",
 					Message:      "value does not match regex pattern `(?i)^[a-z0-9]+$`",
+					ForKey:       true,
 				},
 			),
 		},

tools/protovalidate-conformance/internal/results/result.go

@@ -108,7 +108,11 @@ func (v violationsResult) String() string {
 	errs := v.inner.GetValidationError().GetViolations()
 	_, _ = fmt.Fprintf(bldr, "%d validation error(s)", len(errs))
 	for i, err := range errs {
-		_, _ = fmt.Fprintf(bldr, "\n%s  %2d. %s: %s", resultPadding, i+1, err.FieldPath, err.ConstraintId)
+		forKey := ""
+		if err.ForKey {
+			forKey = " (key)"
+		}
+		_, _ = fmt.Fprintf(bldr, "\n%s  %2d. %s%s: %s", resultPadding, i+1, err.FieldPath, forKey, err.ConstraintId)
 		_, _ = fmt.Fprintf(bldr, "\n%s      %s", resultPadding, err.Message)
 	}
 	return bldr.String()
@@ -128,7 +132,7 @@ func (v violationsResult) IsSuccessWith(other Result, options *harness.ResultOpt
 			return false
 		}
 		for i := 0; i < len(want); i++ {
-			matchingField := want[i].FieldPath == got[i].FieldPath
+			matchingField := want[i].FieldPath == got[i].FieldPath && want[i].ForKey == got[i].ForKey
 			matchingConstraint := want[i].ConstraintId == got[i].ConstraintId
 			if !matchingField || !matchingConstraint {
 				return false