asfdata: enhance where to add: is defined and allow multiple conditions

The where clause currently causes a failure if the target key is not present.
I don't think that should be changed, as it detects typos.
However, some keys may be optional, and it would be useful to be able to exclude those.
Perhaps allow an optional '?' at the end of a key to indicate this.

Also it would be useful to filter entries based on multiple conditions.
I think the most likely need is to support AND.
This could be done by allowing the where attribute to be defined as a list, for example:

```
where:
- status eq active
- homepage?
```