Add support for USING INDEX TABLESPACE

[deeps1991]

Today, there is no way to set tablespace for Unique indexes:

yugabyte=# CREATE TABLE mytable (id int, UNIQUE(id) USING INDEX TABLESPACE mytablespace);
ERROR:  USING INDEX TABLESPACE not supported yet
LINE 1: CREATE TABLE mytable (id int, UNIQUE(id) USING INDEX TABLESP...
                                                 ^
HINT:  See https://github.com/YugaByte/yugabyte-db/issues/1129. Click '+' on the description to raise its priority

Note that this should still not be allowed for primary key index, which is an intrinsic part of the table itself, and should never be associated with a tablespace different from the indexed table.