nanopb: 0.4.6 -> 0.4.8

This commit:
- Bumps the nanopb version
- Adds all runtime configuration options
- Implements proper cross-compilation support which is the main use-case of the library.
- Uses newer `finalAttrs` form of `mkDerivation` to allow for easier attribute overrides.

The cross-compilation support is achieved by splitting the package into two sub-packages
consisting of the build-time generator and the runtime library.

Nanopb explicitely supports this by providing specialized `GENERATOR` and
`RUNTIME` CMake configuration options.

The top-level package uses `propagatedNativeBuildInputs` and `propagatedBuildInputs` to propagate
the sub-packages and also adds convenient symlinks to make certain use cases easier.

== GENERATOR ==

The generator is a mostly ready-to-be-packaged python module tree.
We patch the library to also include the missing `__init__.py` and we
also fix the `PYTHON_INSTDIR` variable to follow best practice and to
prevent the library from attempting to install to a global directory.

We package the python module using `buildPythonPackage` and internally
override python in order to wrap the `nanopb_generator.py` executable.

We do *not* wrap `nanob_generator.py` due to it also being imported directly from python when
used through `protoc-gen-nanopb`.

== RUNTIME ==

The runtime is a simple library that consists of the common
functionality among generated headers/sources.

It is configured through `preprocessor definitions` and consumer projects *must*
be compiled with the same definitions.

This is currently achieved by exposing all configuration options through the top-level overrides and
patching the CMakeLists.txt so that the definitions are added to the to-be-installed
CMake targets as PUBLIC properties.

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