[Spark] Add IdentityColumnsTableFeature (delta-io#2859)

## Description
This PR is part of delta-io#1959

In this PR, we introduce the IdentityColumnsTableFeature to test-only so
that we can start developing with it.

Note, we do not add support to minWriterVersion 6 yet to
properties.defaults.minWriterVersion because that will enable the table
feature outside of testing.

## How was this patch tested?
Existing tests pass. 

## Does this PR introduce _any_ user-facing changes?
No, this is a test-only change.

Author: c27kwan

spark/src/main/scala/org/apache/spark/sql/delta/TableFeature.scala

@@ -354,6 +354,8 @@ object TableFeature {
         TestFeatureWithDependency,
         TestFeatureWithTransitiveDependency,
         TestWriterFeatureWithTransitiveDependency,
+        // Identity columns are under development and only available in testing.
+        IdentityColumnsTableFeature,
         // managed-commits are under development and only available in testing.
         ManagedCommitTableFeature,
         InCommitTimestampTableFeature,
@@ -482,6 +484,16 @@ object ColumnMappingTableFeature
   }
 }
 
+object IdentityColumnsTableFeature
+  extends LegacyWriterFeature(name = "identityColumns", minWriterVersion = 6)
+  with FeatureAutomaticallyEnabledByMetadata {
+  override def metadataRequiresFeatureToBeEnabled(
+      metadata: Metadata,
+      spark: SparkSession): Boolean = {
+    ColumnWithDefaultExprUtils.hasIdentityColumn(metadata.schema)
+  }
+}
+
 object TimestampNTZTableFeature extends ReaderWriterFeature(name = "timestampNtz")
     with FeatureAutomaticallyEnabledByMetadata {
   override def metadataRequiresFeatureToBeEnabled(

spark/src/test/scala/org/apache/spark/sql/delta/DeltaProtocolVersionSuite.scala

@@ -2417,40 +2417,6 @@ trait DeltaProtocolVersionSuiteBase extends QueryTest
     }
   }
 
-  test("can't write to a table with identity columns (legacy protocol)") {
-    withTempDir { dir =>
-      val writerVersion = 6
-      createTableWithProtocol(Protocol(1, writerVersion), dir)
-
-      checkAnswer(
-        sql(s"SELECT * FROM delta.`${dir.getCanonicalPath}`"),
-        spark.range(0).toDF)
-      assert(intercept[InvalidProtocolVersionException] {
-        sql(s"INSERT INTO delta.`${dir.getCanonicalPath}` VALUES (9)")
-      }.getMessage.contains(s" and writer version 6"))
-    }
-  }
-
-  test("can't write to a table with identity columns (table features)") {
-    withTempDir { dir =>
-      val featureName = "identityColumns"
-      createTableWithProtocol(
-        Protocol(
-          TABLE_FEATURES_MIN_READER_VERSION,
-          TABLE_FEATURES_MIN_WRITER_VERSION,
-          readerFeatures = Some(Set.empty),
-          writerFeatures = Some(Set(featureName))),
-        dir)
-
-      checkAnswer(
-        sql(s"SELECT * FROM delta.`${dir.getCanonicalPath}`"),
-        spark.range(0).toDF)
-      assert(intercept[DeltaTableFeatureException] {
-        sql(s"INSERT INTO delta.`${dir.getCanonicalPath}` VALUES (9)")
-      }.getMessage.contains(s"""unsupported by Delta Lake "${io.delta.VERSION}": $featureName"""))
-    }
-  }
-
   def protocolWithFeatures(
       readerFeatures: Seq[TableFeature] = Seq.empty,
       writerFeatures: Seq[TableFeature] = Seq.empty): Protocol = {

spark/src/test/scala/org/apache/spark/sql/delta/DeltaTableFeatureSuite.scala

@@ -146,6 +146,7 @@ class DeltaTableFeatureSuite
         CheckConstraintsTableFeature,
         ChangeDataFeedTableFeature,
         GeneratedColumnsTableFeature,
+        IdentityColumnsTableFeature,
         TestLegacyWriterFeature,
         TestLegacyReaderWriterFeature,
         TestRemovableLegacyWriterFeature,
@@ -215,6 +216,7 @@ class DeltaTableFeatureSuite
           ChangeDataFeedTableFeature,
           CheckConstraintsTableFeature,
           GeneratedColumnsTableFeature,
+          IdentityColumnsTableFeature,
           TestLegacyWriterFeature,
           TestLegacyReaderWriterFeature,
           TestRemovableLegacyWriterFeature,
@@ -243,6 +245,7 @@ class DeltaTableFeatureSuite
               CheckConstraintsTableFeature,
               ChangeDataFeedTableFeature,
               GeneratedColumnsTableFeature,
+              IdentityColumnsTableFeature,
               ColumnMappingTableFeature,
               TestLegacyWriterFeature,
               TestLegacyReaderWriterFeature,
@@ -257,6 +260,7 @@ class DeltaTableFeatureSuite
             CheckConstraintsTableFeature,
             ChangeDataFeedTableFeature,
             GeneratedColumnsTableFeature,
+            IdentityColumnsTableFeature,
             ColumnMappingTableFeature,
             TestLegacyWriterFeature,
             TestLegacyReaderWriterFeature,