## 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 `ExternalCatalogEvent`s using the `SparkListener.onOtherEvent` hook. A more direct approach is add listener directly to the `ExternalCatalog`.

## How was this patch tested?
Added the `ExternalCatalogEventSuite`.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #17710 from hvanhovell/SPARK-20420.