Sequential replies: a single request can have multiple responses

[jakaplan]

This introduces the ability for a client to make a request that the server can respond to arbitrarily many times. It's exposed as either as an AsyncThrowingStream or for closure-based callbacks uses a custom SequentialResult type. This allows a server to essentially stream data to a client and should avoid needing a client to do any sort of polling.

Under the hood the server is now able to make calls to the client which aren't actually XPC replies, but this is intentionally not directly exposed to SecureXPC API users. This limits the conceptual complexity of the framework (e.g. no registration of routes with the client which the server would then send to) while still allowing most bidirectional scenarios.

A few tests were added to ensure the most common scenarios work as expected, but I'm intending to add some more before merging this,

[jakaplan]

@amomchilov Will leave this up for at least a couple days before merging if you'd like to review it beforehand.

[amomchilov]

Heyo, sorry for going dark, I was out on vacay. This is pretty complex stuff, I'll dig into it! I'll let you know if I have any questions/thoughts!

[jakaplan]

No worries! There was a follow up PR after this one that did a bit of cleanup and ensures a transaction is held open, so probably best to look at both (or maybe together? I'm no git expert).