[SPARK-20420][SQL] Add events to the external catalog

[hvanhovell]

What changes were proposed in this pull request?

It is often useful to be able to track changes to the ExternalCatalog. This PR makes the ExternalCatalog emit events when a catalog object is changed. Events are fired before and after the change.

The following events are fired per object:

• Database
  • CreateDatabasePreEvent: event fired before the database is created.
  • CreateDatabaseEvent: event fired after the database has been created.
  • DropDatabasePreEvent: event fired before the database is dropped.
  • DropDatabaseEvent: event fired after the database has been dropped.
• Table
  • CreateTablePreEvent: event fired before the table is created.
  • CreateTableEvent: event fired after the table has been created.
  • RenameTablePreEvent: event fired before the table is renamed.
  • RenameTableEvent: event fired after the table has been renamed.
  • DropTablePreEvent: event fired before the table is dropped.
  • DropTableEvent: event fired after the table has been dropped.
• Function
  • CreateFunctionPreEvent: event fired before the function is created.
  • CreateFunctionEvent: event fired after the function has been created.
  • RenameFunctionPreEvent: event fired before the function is renamed.
  • RenameFunctionEvent: event fired after the function has been renamed.
  • DropFunctionPreEvent: event fired before the function is dropped.
  • DropFunctionPreEvent: event fired after the function has been dropped.

The current events currently only contain the names of the object modified. We add more events, and more details at a later point.

A user can monitor changes to the external catalog by adding a listener to the Spark listener bus checking for ExternalCatalogEvents using the SparkListener.onOtherEvent hook. A more direct approach is add listener directly to the ExternalCatalog.

How was this patch tested?

Added the ExternalCatalogEventSuite.

[SparkQA]

Test build #76007 has finished for PR 17710 at commit 8086e1a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• trait ExternalCatalogEventListener
• trait DatabaseEvent extends ExternalCatalogEvent
• case class CreateDatabasePreEvent(database: String) extends DatabaseEvent
• case class CreateDatabaseEvent(database: String) extends DatabaseEvent
• case class DropDatabasePreEvent(database: String) extends DatabaseEvent
• case class DropDatabaseEvent(database: String) extends DatabaseEvent
• trait TableEvent extends DatabaseEvent
• case class CreateTablePreEvent(database: String, name: String) extends TableEvent
• case class CreateTableEvent(database: String, name: String) extends TableEvent
• case class DropTablePreEvent(database: String, name: String) extends TableEvent
• case class DropTableEvent(database: String, name: String) extends TableEvent
• case class RenameTablePreEvent(
• case class RenameTableEvent(
• trait FunctionEvent extends DatabaseEvent
• case class CreateFunctionPreEvent(database: String, name: String) extends FunctionEvent
• case class CreateFunctionEvent(database: String, name: String) extends FunctionEvent
• case class DropFunctionPreEvent(database: String, name: String) extends FunctionEvent
• case class DropFunctionEvent(database: String, name: String) extends FunctionEvent
• case class RenameFunctionPreEvent(
• case class RenameFunctionEvent(

[rxin]

Merging in master/branch-2.2.