[SPARK-3969][SQL] Optimizer should have a super class as an interface.

Some developers want to replace `Optimizer` to fit their projects but can't do so because currently `Optimizer` is an `object`.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #2825 from ueshin/issues/SPARK-3969 and squashes the following commits:

abbc53c [Takuya UESHIN] Re-rename Optimizer object.
4d2e1bc [Takuya UESHIN] Rename Optimizer object.
9547a23 [Takuya UESHIN] Extract abstract class from Optimizer for developers to be able to replace Optimizer.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -28,7 +28,9 @@ import org.apache.spark.sql.catalyst.plans.logical._
 import org.apache.spark.sql.catalyst.rules._
 import org.apache.spark.sql.catalyst.types._
 
-object Optimizer extends RuleExecutor[LogicalPlan] {
+abstract class Optimizer extends RuleExecutor[LogicalPlan]
+
+object DefaultOptimizer extends Optimizer {
   val batches =
     Batch("Combine Limits", FixedPoint(100),
       CombineLimits) ::

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/ExpressionOptimizationSuite.scala

@@ -30,7 +30,7 @@ class ExpressionOptimizationSuite extends ExpressionEvaluationSuite {
       expected: Any,
       inputRow: Row = EmptyRow): Unit = {
     val plan = Project(Alias(expression, s"Optimized($expression)")() :: Nil, NoRelation)
-    val optimizedPlan = Optimizer(plan)
+    val optimizedPlan = DefaultOptimizer(plan)
     super.checkEvaluation(optimizedPlan.expressions.head, expected, inputRow)
   }
-}
\ No newline at end of file
+}

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

@@ -29,7 +29,7 @@ import org.apache.spark.sql.catalyst.ScalaReflection
 import org.apache.spark.sql.catalyst.analysis._
 import org.apache.spark.sql.catalyst.dsl.ExpressionConversions
 import org.apache.spark.sql.catalyst.expressions._
-import org.apache.spark.sql.catalyst.optimizer.Optimizer
+import org.apache.spark.sql.catalyst.optimizer.{Optimizer, DefaultOptimizer}
 import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.catalyst.rules.RuleExecutor
 import org.apache.spark.sql.catalyst.types.DataType
@@ -68,7 +68,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
     new Analyzer(catalog, functionRegistry, caseSensitive = true)
 
   @transient
-  protected[sql] val optimizer = Optimizer
+  protected[sql] lazy val optimizer: Optimizer = DefaultOptimizer
 
   @transient
   protected[sql] val sqlParser = {