Add support min/max functions

[atulkc]

As part of supporting aggregate functions add support for following functions:

MIN()
MAX()

Add min and max tag that can be used in selectClause structs as follows:

type TestSoqlStruct struct {
	SelectClause NestedStruct      `soql:"selectClause,tableName= Contact"`
	GroupBy        []string                 `soql:"groupByClause"`
}

type NestedStruct struct {
        FirstName     string `soql:"selectColumn,fieldName=FirstName"`
        LastName     string `soql:"selectColumn,fieldName=LastName"`
	Count            int `soql:"min,fieldName= CreatedDate"`
}

soqlStruct := TestSoqlStruct{
   GroupBy: []string{"FirstName", "LastName"},
}
soqlQuery, err := Marshal(soqlStruct)
if err != nil {
    fmt.Printf("Error in marshaling: %s\n", err.Error())
}
fmt.Println(soqlQuery)

This should result in SOQL as follows

SELECT FirstName, LastName, MIN(CreatedDate) FROM Contact GROUP BY FirstName, LastName'

Note that if fieldName is not specified then return ErrInvalidTag.

Same should apply for max tag as well.

Allow min and max to be used only for supported primitive data types