## What changes were proposed in this pull request?
We're building a data lineage tool in which we need to monitor the metadata changes in ExternalCatalog, current ExternalCatalog already provides several useful events like "CreateDatabaseEvent" for custom SparkListener to use. But still there's some event missing, like alter database event and alter table event. So here propose to and new ExternalCatalogEvent.
## How was this patch tested?
Enrich the current UT and tested on local cluster.
CC hvanhovell please let me know your comments about current proposal, thanks.
Author: jerryshao <sshao@hortonworks.com>
Closes#19649 from jerryshao/SPARK-22405.
6793a3dac0