Make comment support conditional on fn_listextendedproperty availability

Make comment support conditional on fn_listextendedproperty availability

The newly added comment reflection and rendering capability of the MSSQL
dialect, added in :ticket:`7844`, will now be disabled by default if it
cannot be determined that an unsupported backend such as Azure Synapse may
be in use; this backend does not support table and column comments and does
not support the SQL Server routines in use to generate them as well as to
reflect them. A new parameter ``supports_comments`` is added to the dialect
which defaults to ``None``, indicating that comment support should be
auto-detected. When set to ``True`` or ``False``, the comment support is
either enabled or disabled unconditionally.

Fixes: #9142
Change-Id: Ib5cac31806185e7353e15b3d83b580652d304b3b