Fix lint for repeated protovalidate CEL expression (#2824)

This PR fixes linting for repeated fields with CEL expressions. Adds a
new test case for lists (and maps) to ensure a valid expression passes.
Small fix to set the type 'this' to the list element type.

Before:
```
testdata/protovalidate_rules/cel_field.proto:126:9:(buf.validate.field).repeated.items.cel.expression on field "allow_cidr" fails to compile: found no matching overload for 'isIpPrefix' applied to 'list(string).(int, bool)'
         | this.isIpPrefix(4, true)
         | ...............^
```

private/bufpkg/bufcheck/buflint/internal/buflintvalidate/cel.go

@@ -86,7 +86,7 @@ func checkCELForField(
 	celEnv, err = celEnv.Extend(
 		append(
 			celext.RequiredCELEnvOptions(fieldDescriptor),
-			cel.Variable("this", celext.ProtoFieldToCELType(fieldDescriptor, false, false)),
+			cel.Variable("this", celext.ProtoFieldToCELType(fieldDescriptor, false, fieldDescriptor.IsList())),
 		)...,
 	)
 	if err != nil {

private/bufpkg/bufcheck/buflint/testdata/protovalidate_rules/cel_field.proto

@@ -119,4 +119,29 @@ message Foo {
       expression: "this * this"
     }
   ];
+  // Valid CEL expressions for repeated fields.
+  repeated string allow_cidr = 14 [(buf.validate.field).repeated = {
+    min_items: 1,
+    items: {
+      cel: [
+        {
+          id: "ip_prefix",
+          message: "value must be IPv4 prefix",
+          expression: "this.isIpPrefix(4, true)",
+        }
+      ],
+    },
+  }];
+  // Valid CEL expressions for map fields.
+  map<string, bool> allow_cidr_map = 15 [(buf.validate.field).map = {
+    keys: {
+      cel: [
+        {
+          id: "ip_prefix",
+          message: "key must be IPv4 prefix",
+          expression: "this.isIpPrefix(4, true)",
+        }
+      ],
+    },
+  }];
 }