Add and test support for handling simple `attributeExpression`

[sarthak77]

In response to this discussion, adding support for simple attribute. Eventually, this will replace the columnIdentifier but currently adding support so that requests containing any of the columnIentifier and attributeExpression will work.

e.g In a correct request, a subpath will not be present and attribute is mapped to a simple long column, QS can serve it.

AttributeExpression {
attributeId: API_TRACE.startTime";
}

Dividing it into following sub tasks :

• Add support for simple attributes in QueryRequestToPinotSQLConverter.java
• Add relevant tests in QueryRequestToPinotSQLConverterTest.java