Add support for Partitioning and Sample pages on mysql

Add support for Partitioning and Sample pages on mysql

Add support for Partitioning and Sample pages on MySQL and MariaDB
reflected options.
The options are stored in the table dialect options dictionary, so
the following keyword need to be prefixed with ``mysql_`` or ``mariadb_``
depending on the backend.
Supported options are:

* ``stats_sample_pages``
* ``partition_by``
* ``partitions``
* ``subpartition_by``

These options are also reflected when loading a table from database,
and will populate the table :attr:`_schema.Table.dialect_options`.
Pull request courtesy of Ramon Will.

Fixes: #4038
Closes: #5536
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5536
Pull-request-sha: f8852cabe15c9a91de85d27980988051f7a1306d

Change-Id: I69b60576532af04c725c998e9e8fec6e2040b149