…tter of its path is slash in create/alter table

After apache#28527, we change to create table under the database location when the table location is relative. However the criteria to determine if a table location is relative/absolute is `URI.isAbsolute`, which basically checks if the table location URI has a scheme defined. So table URIs like `/table/path` are treated as relative and the scheme and authority of the database location URI are used to create the table. For example, when the database location URI is `s3a://bucket/db`, the table will be created at `s3a://bucket/table/path`, while it should be created under the file system defined in `SessionCatalog.hadoopConf` instead.

This change fixes that by treating table location as absolute when the first letter of its path is slash.

This also applies to alter table.

This is to fix the behavior described above.

Yes. When users try to create/alter a table with a location that starts with a slash but without a scheme defined, the table will be created under/altered to the file system defined in `SessionCatalog.hadoopConf`, instead of the one defined in the database location URI.

Updated unit tests.

Closes apache#35462 from bozhang2820/spark-31709.

Authored-by: Bo Zhang <bo.zhang@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>