lib: make diagnostics_channel async iterable
This is just an idea for a possibly helpful extension to the diagnostics_channel feature in #34895. Not sure how valuable this is, so I've pulled it out to a separate PR where people can
Depends on: #34895
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines