timestamp logical types from numerical types

[the4thamigo-uk]

Currently, if I try to convert an int in a native golang type to a timestamp-millis in AVRO, I get an error. It seems like only time.Time can be converted to timestamp-millis. This seems quite restrictive, as all the numeric types should be able to be coerced to int64 and then to timestamp-millis.

I have a fix for this, would you be amenable to a PR for this behaviour?