Introduce interface to export InvocationStatus

[slinkydeveloper]

Fix #517

[slinkydeveloper]

cc @igalshilman

[tillrohrmann]

Thanks for creating this PR @slinkydeveloper. I had a few questions about the guarantees of the Weak pointer and what the user of InvokerStatusReader should see.

[igalshilman]

@slinkydeveloper The API looks exactly what we would need at the query side!

We might add/remove fields over time to fine tune the table structure that we'll expose.

One question tho, to you and @tillrohrmann - what do you think about moving this to common, so that we don't add an explicit dependency to the invoker?
Or alternatively, if possible create a thin api crate for the invoker - wdyt?

[slinkydeveloper]

Or alternatively, if possible create a thin api crate for the invoker - wdyt?

Probably we'll do the latter. See #392 and #420 and #528

[igalshilman]

Ok then, I will wait until this will be moved to the common crate or an API crate.

[tillrohrmann]

LGTM. +1 for merging.