add method to retrieve count of external files

[stevendanna]

This allows callers to operate differently if the database contains external files that might be slower to read than expected.

[cockroach-teamcity]

This change is 

[dt]

I think the second return value of EstimateDiskUsageByBackingType could probably serve the same purpose as this unless that method is too expensive?

[stevendanna]

I'll start with that and only move forward with this function if I start seeing it actually being a problem.