[SQL] Is a config modifiable at runtime

In the PR, I propose to extend `RuntimeConfig` by new method `isModifiable()` which returns `true` if a config parameter can be modified at runtime (in notebooks). For static SQL and core parameters, the method returns `false`.

python/pyspark/sql/conf.py

@@ -63,6 +63,12 @@ def _checkType(self, obj, identifier):
             raise TypeError("expected %s '%s' to be a string (was '%s')" %
                             (identifier, obj, type(obj).__name__))
 
+    @ignore_unicode_prefix
+    @since(2.4)
+    def isModifiable(self, key):
+        """Is the configuration property modifiable or not."""
+        return self._jconf.isModifiable(key)
+
 
 def _test():
     import os

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -1892,4 +1892,8 @@ class SQLConf extends Serializable with Logging {
     }
     cloned
   }
+
+  def isModifiable(key: String): Boolean = {
+    sqlConfEntries.containsKey(key) && !staticConfKeys.contains(key)
+  }
 }

sql/core/src/main/scala/org/apache/spark/sql/RuntimeConfig.scala

@@ -132,6 +132,13 @@ class RuntimeConfig private[sql](sqlConf: SQLConf = new SQLConf) {
     sqlConf.unsetConf(key)
   }
 
+  /**
+   * Can the configuration property be modified at runtime.
+   *
+   * @since 2.4.0
+   */
+  def isModifiable(key: String): Boolean = sqlConf.isModifiable(key)
+
   /**
    * Returns whether a particular key is set.
    */

sql/core/src/test/scala/org/apache/spark/sql/RuntimeConfigSuite.scala

@@ -54,4 +54,15 @@ class RuntimeConfigSuite extends SparkFunSuite {
       conf.get("k1")
     }
   }
+
+  test("is a config parameter modifiable") {
+    val conf = newConf()
+
+    // SQL configs
+    assert(!conf.isModifiable("spark.sql.sources.schemaStringLengthThreshold"))
+    assert(conf.isModifiable("spark.sql.streaming.checkpointLocation"))
+    // Core configs
+    assert(!conf.isModifiable("spark.task.cpus"))
+    assert(!conf.isModifiable("spark.executor.cores"))
+  }
 }