[c-lightning] Plugins/passthrough for next version?

[Corné Plooy]

> We already have number of tools that use the Driver or the Sidecar
> model, and we have at least one PR that would give us a Callback
> extension point (PR #1715). 
To clarify: PR #1715 just adds some functionality to sendpay, which is
to be used in combination with a future callback mechanism (not in PR
#1715).

PR #1686 is an older PR that contains both parts, but after some thought
and after changing the callback design, I decided to split it up into PR
#1715 and a future PR that is waiting for PR #1715 to be merged. For a
preview of the future PR (including the callback mechanism), see:

https://github.com/bitonic-cjp/lightning/tree/custom_router

CJP