Add clock offset

[Erfa]

This library is working great for me, but I have an idea for a small improvement. In my case I want to use it on timestamps from a server. There's a risk that the client's clock diverges from the server timestamps and shows incorrect values.

I am able to get an estimate on how much the server clock differs from the client clock.

Suggested solution: add a parameter clockOffset that takes this difference either in milliseconds or as a Duration, and uses it when calculating how long ago something happened.

I initially thought that this was what the clock parameter did, but it seems like that one just sends in a fixed time. Another solution would be to change the clock to accept a lambda instead to be called every time it needs the current time, that might be more flexible.