Add is_replaying attribute to WFActivation

[bergundy]

Why?

This is required in order for lang SDKs to be able to dedupe log messages and metrics.
See https://github.com/temporalio/proposals/blob/master/node/logging-and-metrics-for-user-code.md

[bergundy]

I tested this and only got false, I see the TODO comment in the code, we'd need to address this for it to actually be useful.

[bergundy]

I'm merging this despite the incorrectness of the value.
Let's prioritize fixing this since a lot of functionality relies on the is_replaying flag.
One thing I'm not sure of is if this flag should be set per activation or per activation job.
@Sushisource @mfateev @vitarb WDYT?