Add rudimentary UNION support to the QueryBuilder

[sbuerk]

Q	A
Type	feature
Fixed issues	#6368

Summary

The UNION operator is used to combine the result-set
of two or more SELECT statements, which all database
vendors supports with usual specialities for each.

Still, there is a common shared subset which works for
all of them:

    SELECT column_name(s) FROM table1
    WHERE ...

    UNION <ALL | DISTINCT>

    SELECT column_name(s) FROM table2
    WHERE ...

    ORDER BY ...
    LIMIT x OFFSET y

with shared common requirements:

• Each SELECT must return the same fields
  in number, naming and order.

• Each SELECT must not have ORDER BY,
  expect MySQL allowing it to be used as sub
  query expression encapsulated in parentheses.

Taking the shared requirements and working behaviour,
it is possible to provide a generic support to the
QueryBuilder with a minimalistic surface addition now,
and following methods are added:

• union(string|QueryBuilder ...$unionParts) and
  addUnion(string|QueryBuilder ...$unionParts) to
  create a UNION query retrieving unique rows
• unionAll(string|QueryBuilder ...$unionParts) and
  addUnionAll(string|QueryBuilder ...$unionParts) to
  create a UNION ALL query retrieving eventually
  duplicated rows.

This follows the generic logic as select(...) and
addSelect(...) along with introducing new internal
QueryType::UNION_DISTINCT and QueryType::UNION_ALL
enum cases.

Additional to the consideration to allow SQL strings
and QueryBuilder for union(), unionAll(), addUnion()
and addUnionAll() and minimize the direct handling
of miss-configuration to the number of provided parts
and let other issues like the field (order, count, naming)
or not allowed order by handling to the database itself.
With that, vendor specific implementation can be done if
required.

Technically, the SQL build process is dispatched to a
DefaultUnionSQLBuilder along with an UnionSQLBuilder
interface, which also allows application to implement
custom behaviour if required.

Example:

$platform = $connection->getDatabasePlatform();
$qb       = $>connection->createQueryBuilder();
$select10 = $platform->getDummySelectSQL('2 as field_one');
$select20 = $platform->getDummySelectSQL('1 as field_one');
$qb->union($select10,    /*$select11, $select12, ... */)
   ->addUnion($select20, /*$select21, $select22, ... */)
   ->setMaxResults(1)
   ->setFirstResult(1)
   ->orderBy('field_one', 'ASC');
$rows = $qb->executeQuery()->fetchAllAssociative();

Unit and functional tests are added to demonstrate the
implementation and cover it for future changes.

Resolves: #6368