Fix zstd-dll build missing dependencies

[yoniko]

Fixes zstd-dll build (#3492 ):

• Adds pool.o and threading.o dependency to the zstd-dll target
• Moves custom allocation functions into header to avoid needing to add dependency on common.o
• Adds test target for zstd-dll
• Adds github workflow for test zstd-dll

[yoniko]

@Cyan4973 - Is there any issue with adding this to zstd_internal.h?

[Cyan4973]

Well, zstd_internal.h is used everywhere within libzstd, so this change can impact a lot of places.
I'm not sure if I can understand all the implications.

[Cyan4973]

This PR manages to get zstd-dll recipe working by adding 2 units, threading.c and pool.c.
This is an improvement over a naive solution which requires 4 units, including zstd_common.c and error_private.c.

To achieve that goal, this PR moves customMalloc() and co from zstd_common.c to zstd_internal.h.
This operation requires making zstd_deps.h a dependency of zstd_internal.h, and therefore, transitively, a dependency of almost all source files inside libzstd.

Now, I'm not exactly pleased about this dependency story.
But on the other hand, I don't think we lose too much in this PR specifically.

zstd_common.c is not terribly specific,
moving functions from there to zstd_internal.h doesn't improve the situation, but it does not make it significantly worse either.

Bundling more definitions within zstd_internal.h will transitively increase compilation load time of all units depending on it. But the increased workload is likely very light, so it's unlikely to have a big impact.

All in all, it seems this is an acceptable trade off.

Longer term, I'm not pleased to see zstd_internal.h employed as a "bag of everything" which transitively includes a ton of other stuffs. Initially, this file was designed to hold common information that should not be duplicated between compression and decompression, such as the Frame ID number. It has largely deviated from this original scope, and I think this doesn't respect the libzstd model of minimal dependency at each step. Hence, I'll be pleased to see this bundle gradually separated into more specific packets that are included only when needed.

But that's a topic for a different effort, and since this PR is focused on zstd-dll, it wouldn't be fair to address that here.

[yoniko]

@Cyan4973 - all tests pass. Please see most recent changes, if it looks good to you we can merge.