feat(backends): add sglang (#9359)

* feat(backends): add sglang

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* fix(sglang): force AVX-512 CXXFLAGS and disable CI e2e job

sgl-kernel's shm.cpp uses __m512 AVX-512 intrinsics unconditionally;
-march=native fails on CI runners without AVX-512 in /proc/cpuinfo.
Force -march=sapphirerapids so the build always succeeds, matching
sglang upstream's docker/xeon.Dockerfile recipe.

The resulting binary still requires an AVX-512 capable CPU at runtime,
so disable tests-sglang-grpc in test-extra.yml for the same reason
tests-vllm-grpc is disabled. Local runs with make test-extra-backend-sglang
still work on hosts with the right SIMD baseline.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* fix(sglang): patch CMakeLists.txt instead of CXXFLAGS for AVX-512

CXXFLAGS with -march=sapphirerapids was being overridden by
add_compile_options(-march=native) in sglang's CPU CMakeLists.txt,
since CMake appends those flags after CXXFLAGS. Sed-patch the
CMakeLists.txt directly after cloning to replace -march=native.

---------

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>