Folder structure refactor

[DvirDukhan]

this PR introduces:

1. reorganization of the folder structure into a more logical order.
2. cosmetics - better include statement readability - order and syntax
3. replaced include guard with pragma once
4. removed unused files

The purpose of this PR is to be a building block for enabling generic DAG ops, and as such enable DAG plugin API, for different purpose extensions (e.g. native data types support. As such, we probably like to merge this to 1.2 in the purpose of having a smaller code delta compared to master or 1.4

[codecov]

Codecov Report

Merging #629 (7c2c2f0) into master (d6d98ef) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #629   +/-   ##
=======================================
  Coverage   78.25%   78.25%           
=======================================
  Files          41       41           
  Lines        6535     6535           
=======================================
  Hits         5114     5114           
  Misses       1421     1421           

Impacted Files	Coverage Δ
src/backends/backends.c	62.89% <ø> (ø)
src/backends/libtflite_c/tflite_c.cpp	58.17% <ø> (ø)
src/backends/libtorch_c/torch_c.cpp	57.89% <ø> (ø)
...ckends/libtorch_c/torch_extensions/torch_redis.cpp	84.44% <ø> (ø)
...backends/libtorch_c/torch_extensions/torch_redis.h	88.88% <ø> (ø)
src/backends/onnxruntime.c	73.10% <ø> (ø)
src/backends/tensorflow.c	70.50% <ø> (ø)
src/backends/tflite.c	64.05% <ø> (ø)
src/backends/torch.c	80.25% <ø> (ø)
src/config/config.c	55.55% <ø> (ø)
... and 21 more

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[lantiga]

The new structure looks good to me, it makes it easier to navigate the code.

[alonre24]

Looks good!