[dc-rules] add rule for sql structs

[peakle]

Is your feature request related to a problem? Please describe.
add rule which will check sql structs with pointers to basic types (string, int, bool, etc) and will recommend change it to sql.Null* type. So we can decrease load on heap and increase runtime speed

POC:

type RowHeap struct {
	Str *string `db:"str"`
}

type RowStack struct {
	Str sql.NullString `db:"str"`
}

BenchmarkResults https://github.com/peakle/meetup/blob/master/go-optimizations/bench2_test.go:

RowHeap:
BenchmarkRowHeap-16           59960768                19.82 ns/op           16 B/op          1 allocs/op
BenchmarkRowHeap-16           61210947                21.10 ns/op           16 B/op          1 allocs/op
BenchmarkRowHeap-16           49640668                28.02 ns/op           16 B/op          1 allocs/op

RowStack:
BenchmarkRowStack-16           1000000000               0.7385 ns/op          0 B/op          0 allocs/op
BenchmarkRowStack-16           1000000000               0.7114 ns/op          0 B/op          0 allocs/op
BenchmarkRowStack-16           1000000000               0.7178 ns/op          0 B/op          0 allocs/op