Provide a few columnExpression() with aggregates

[Pierre]

Most commonly used aggregate (or not) expressions are the following as of now:

• ->columnExpression('true') and ->columnExpression('false')
• ->columnExpression('1')
• ->columnExpression("count(some_column)") and ->columnExpression("count(*)")
• ->columnExpression(ExpressionRaw::create('array_agg(some_column)'))
• ->columnExpression(ExpressionRaw::create('coalesce(col1, col2, ...)'), 'alias')

[Pierre]

Duplicate of #65