Async read implementation for usb serial jtag

[jneem]

This is an attempt at adding an async read implementation for usb_serial_jtag to complement the existing async write implementation. I'm pretty unsure about whether I'm doing the interrupt handling correctly, but I did at least check that a basic implementation works.

• The code compiles without errors or warnings.
• All examples work.
• cargo fmt was run.
• Your changes were added to the CHANGELOG.md in the proper section.
• You updated existing examples or added examples (if applicable).
• Added examples are checked in CI

Nice to have

• You add a description of your work to this PR.
• You added proper docs for your newly added features and code.

[jessebraham]

Thanks for the PR, apologies for taking so long to get to this! I will try to get this tested and reviewed in the next couple days.

[jessebraham]

I needed to rebase this branch in order to get it building, however after doing so it seems to be working as expected!

So, if you wouldn't mind rebasing (there is unfortunately a conflict with CHANGELOG.md, anyway) then I think this is probably good to go!

Additionally, if you're willing to add examples for the other chips that would be great, but I can do so in a subsequent PR so it's not huge deal.

[jneem]

Ok, I've added examples for all c6, h2, and s3 (basically all the ones that already had a non-embassy usb_serial_jtag example). They're untested, though, because I don't have any of them.

I also took the liberty of removing the scary comment from s3's usb_serial_jtag example because that issue is fixed, apparently.

[jessebraham]

LGTM, thanks again for this!