Add support for configuring expected frequency / tolerance of external diagnostic publishers

[bmchalenv]

Currently external topics do not have their expected frequency / tolerance configurable. This could be done with parameters. An implementation exists here #21 .

[sgillen]

Just to add some context, we went back and forth on this and I decided that this was adding to much code for a relatively niche feature that we don't have a use case for yet. But if there is enough user demand or compelling use cases we can add it.