filter separately in type and subname in records

[dergeberl]

Thanks for the great library!

Do you think it is possible to adjust this filter section, to can filter on a subname and a type separately? For example this is needed to filter all A records over all subnames.

desec/records.go

Lines 49 to 54 in b4c3e4a

	if filter != nil {
	query := endpoint.Query()
	query.Set("type", filter.Type)
	query.Set("subname", filter.SubName)
	endpoint.RawQuery = query.Encode()
	}

My first idea was to check if the value of Type and SubName is not empty ("") before run query.Set(). But in case of SubName this will not work like expected because with the empty filter you find the records based on the basedomain.

desec/records.go

Lines 25 to 28 in b4c3e4a

	type RRSetFilter struct {
	Type string
	SubName string
	}

Maybe an idea is to set SubName (in RRSetFilter) to a pointer so it can be nil, and we can react in this to not set it as a filter.