[SPARK-15583][SQL] Disallow altering datasource properties

## What changes were proposed in this pull request?

Certain table properties (and SerDe properties) are in the protected namespace `spark.sql.sources.`, which we use internally for datasource tables. The user should not be allowed to

(1) Create a Hive table setting these properties
(2) Alter these properties in an existing table

Previously, we threw an exception if the user tried to alter the properties of an existing datasource table. However, this is overly restrictive for datasource tables and does not do anything for Hive tables.

## How was this patch tested?

DDLSuite

Author: Andrew Or <andrew@databricks.com>

Closes #13341 from andrewor14/alter-table-props.