Introduce field and vector extraction protocols

docs/source/api/lang.rst

docs/source/usage/api_modelling.md

docs/source/usage/how_to_composer.md

src/pystencilssfg/composer/basic_composer.py

src/pystencilssfg/ir/postprocessing.py

src/pystencilssfg/lang/__init__.py

src/pystencilssfg/lang/cpp/std_mdspan.py

src/pystencilssfg/lang/cpp/std_span.py

src/pystencilssfg/lang/cpp/std_tuple.py

src/pystencilssfg/lang/cpp/std_vector.py

src/pystencilssfg/lang/cpp/sycl_accessor.py

src/pystencilssfg/lang/expressions.py

src/pystencilssfg/lang/extractions.py

tests/generator_scripts/source/MdSpanFixedShapeLayouts.py

tests/ir/test_postprocessing.py