syntax extensions (patch 1)

syntax extensions (patch 1)

Added the ability to create custom SQL constructs that can define new
clauses within SELECT, INSERT, UPDATE, and DELETE statements without
needing to modify the construction or compilation code of of
:class:`.Select`, :class:`.Insert`, :class:`.Update`, or :class:`.Delete`
directly.  Support for testing these constructs, including caching support,
is present along with an example test suite.  The use case for these
constructs is expected to be third party dialects for NewSQL or other novel
styles of database that introduce new clauses to these statements.   A new
example suite is included which illustrates the ``QUALIFY`` SQL construct
used by several NewSQL databases which includes a cachable implementation
as well as a test suite.

Since these extensions start to make it a bit crowded with how many
kinds of "options" we have on statements, did some naming /
documentation changes with existing constructs on Executable, in
particular to distinguish ExecutableOption from SyntaxExtension.

Fixes: #12195
Change-Id: I4a44ee5bbc3d8b1b640837680c09d25b1b7077af