Support IF EXISTS/IF NOT EXISTS for DDL constructs

Support IF EXISTS/IF NOT EXISTS for DDL constructs

Added parameters :paramref:`_ddl.CreateTable.if_not_exists`,
:paramref:`_ddl.CreateIndex.if_not_exists`,
:paramref:`_ddl.DropTable.if_exists` and
:paramref:`_ddl.DropIndex.if_exists` to the :class:`_ddl.CreateTable`,
:class:`_ddl.DropTable`, :class:`_ddl.CreateIndex` and
:class:`_ddl.DropIndex` constructs which result in "IF NOT EXISTS" / "IF
EXISTS" DDL being added to the CREATE/DROP. These phrases are not accepted
by all databases and the operation will fail on a database that does not
support it as there is no similarly compatible fallback within the scope of
a single DDL statement.  Pull request courtesy Ramon Williams.

Fixes: #2843
Closes: #5663
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5663
Pull-request-sha: 748b8472345d96efb446e2a444fbe020b313669f

Change-Id: I6a2b1f697993ed49c31584f0a31887fb0a868ed3