feat[next]: Introduce FunctionField

[samkellerhals]

Description

Introduces a FunctionField which can also be used to make constant fields.

Requirements

• All fixes and/or new features come with corresponding tests.
• Important design decisions have been documented in the approriate ADR inside the docs/development/ADRs/ folder.

If this PR contains code authored by new contributors please make sure:

• All the authors are covered by a valid contributor assignment agreement provided to ETH Zurich and signed by the employer if needed.
• The PR contains an updated version of the AUTHORS.md file adding the names of all the new contributors.

[samkellerhals]

@havogt Should be ready for another review now. Only thing I am still missing is to make mypy happy on the three issues below:

src/gt4py/next/embedded/function_field.py:222:12: error: Cannot instantiate abstract class "FunctionField" with abstract attributes "__gt_builtin_func__", "__gt_dims__", "__gt_origin__" and "dtype"  [abstract]
src/gt4py/next/embedded/function_field.py:232:12: error: Cannot instantiate abstract class "FunctionField" with abstract attributes "__gt_builtin_func__", "__gt_dims__", "__gt_origin__" and "dtype"  [abstract]
src/gt4py/next/embedded/function_field.py:235:58: error: Argument 2 to "register_builtin_func" of "FieldBuiltinFuncRegistry" has incompatible type "Callable[[FunctionField[Any, Any], tuple[Dimension, ...]], FunctionField[Any, Any]]"; expected "Callable[[Field[Any, Any] | gt4py_defs.ScalarT, tuple[Dimension, ...]], Field[Any, Any] | Any] | None"  [arg-type]

[havogt]

Sorry, had these pending comments, some of them should be obsolete now.

[havogt]

As you already pointed out, there are some tests that should be run on all fields (e.g. all fields need to support all operators and fbuiltins). I think it makes sense to try to extract these into a test that works universally.
I didn't think about the problem, but maybe just compare a numpy reference against field.ndarray could be good enough.

Of course, there are also other tests that should stay in the concrete field test file.

[samkellerhals]

I tried cleaning the tests up a bit by introducing a test in nd_array_field which tests the unary builtins for all of the field types (BaseNdarrayField and FunctionField). I made sure to share the operator fixtures across the ndarray and function field tests to ensure that the same operators are tested in both instances, see 2a18ccc

Other than that I don't know how much value there is to refactor the code even further, do you want to also have some sort of global test for all of the operators?