Add sendAsync functions to web push service to support non-blocking calls

[svenopdehipt]

I did add non blocking async (suspend) function as an alternative to the blocking non async functions. I had to add one dependency to support this workflow (kotlinx-coroutines-core), which is an official Kotlin dependency from JetBrains. This did increase the jar file by 2 KB (from 24 KB to 26 KB), which shouldn't be a large impact (this should still be considered lightweight). Only the part with the zero dependency wouldn't be true, but it would only be 1 first party dependency. If you don't like this, you can just reject my request.

[svenopdehipt]

I have now removed the third party dependency.