Decode AVX512 UTF8

[Nick-Nuon]

This PR is for the AVX512 decode for the UTF8 . The tests pass but unfortunately the perfomance is so-so:

Method	FileName	Mean	Error	StdDev	Speed (GB/s)
AVX2DecodingRealDataUTF8	data/email/	308.9 us	0.69 us	0.46 us	6.40
AVX512DecodingRealDataUTF8	data/email/	3,408.5 us	10.40 us	6.19 us	.58
SSEDecodingRealDataUTF8	data/email/	409.3 us	0.24 us	0.13 us	4.83
DotnetRuntimeSIMDBase64RealDataUTF8	data/email/	648.1 us	0.80 us	0.42 us	3.05

Of particular note , I couldn't find a reference to _mm512_maskz_compress_epi8 in the documentation so I assume its not exposed
: I used the same routine to compress as in SSE and AVX2 to replace it.

Still working on tuning it but I'll probably ping soon.

[lemire]

Ok. Good. :-)

[lemire]

I think I see why it is not faster. I should be able to fix that!!!

[lemire]

Ah. No. We can't do it yet because they have not exposed the necessary functionality yet:

dotnet/runtime#88946