applets: add a bi-directional bulk_speed_test applet

[antoinevg]

This PR adds a new applet that is able to form a bi-directional bulk speed test.

I don't have access to SuperSpeed hardware so I thought it best to implement this separately to the bulk_in_speed_test.py applet for now.

New features added:

• Addition of a new OUT endpoint. (Added bonus: the speed test device will now enumerate under macOS)
• Vendor request handler to manage the switch between IN/OUT test.
• Data sent by the IN test is now an 8-bit counter rather than zeros.
• OUT test mirrors received data on device LED's and PMOD A.

There is one area where I am not 100% sure I'm doing the right thing in the gateware implementation and that is the logic for when a given IN or OUT test is inactive:

IN test:

https://github.com/antoinevg/luna/blob/791498f1eabb3632453770a5a3c5f7d3876e3c2a/applets/bulk_speed_test.py#L229-L236

OUT test:

https://github.com/antoinevg/luna/blob/791498f1eabb3632453770a5a3c5f7d3876e3c2a/applets/bulk_speed_test.py#L266-L270

[martinling]

A couple of comments:

1. I don't think the vendor requests are necessary. Since the IN endpoint generates endless data from nowhere, and the OUT endpoint accepts endless data going nowhere, there's no need to explicitly start/stop them, nor to have only one in use at once.

2. To avoid duplicating the implemetation, it may be worth rebasing this on Move speed test gateware into library #207, which moves the implementations out of bulk_in_speed_test.py and splits them up into SuperSpeed and non-SuperSpeed versions.

[antoinevg]

I've removed the vendor request handler without any ill-effects, tx!

Let's wait for #207 to get merged into main and then I'll update the PR to match the new code layout.

[antoinevg]

I've rebased the PR to take #207 into account.

Also:

• Renamed applets/bulk_in_speed_test.py to applets/bulk_speed_test.py
• Added an exclusion for SuperSpeed devices so that only the IN test gets performed for them.
• Reworked the logic for bulk_speed_test.__main__ to make it a little less verbose.

[miek]

This looks good to me! There's just a minor fix needed that I've added inline.