[roll] Roll fuchsia [driver_runtime] Move some dispatcher internals out

This moves mostly bookkeeping types that are only important to the
dispatcher itself out into their own file. None of them are really worth
having a file of their own to themselves, but this makes it easier to
see them, and simplifies the dispatcher type definition in dispatcher.h
significantly.

There should be no substantial changes to code here, only things being
moved around. The one small exception is making it so the
DispatcherInterface doesn't need to be a friend to assert the canary
value in the dispatcher object. Further improvements to encapsulation
are deferred so they don't get lost in the noise.

Original-Original-Bug: 520083040
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1638396
Original-Original-Revision: 3f2ba90ae15df9416024b07fc1dc5531054d8708
GitOrigin-RevId: 4453ddefa9c8bccb5d7a22501c1c9b04da5d1b24
Change-Id: Idf25b600238257d1f1d88a2224410d2510b0b964
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1641040
Cr-Commit-Position: refs/heads/main@{#194233}