nix: create nix flake and libwaku-android-arm64 target

* android-ndk is added
* in the derivation, system nim is default but one can change it to
  nimbus-build-system
* special script for creating nimble links, necessary for the
  compilation to succeed.

Referenced issue:
* https://github.com/waku-org/nwaku/issues/3232

scripts/generate_nimble_links.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+
+# This script is used for building Nix derivation which doesn't allow Git commands.
+# It implements similar logic as $(NIMBLE_DIR) target in nimbus-build-system Makefile.
+
+create_nimble_link_script_path="$(pwd)/${BUILD_SYSTEM_DIR}/scripts/create_nimble_link.sh"
+
+process_gitmodules() {
+  local gitmodules_file="$1"
+  local gitmodules_dir=$(dirname "$gitmodules_file")
+  
+  # Extract all submodule paths from the .gitmodules file
+  grep "path" $gitmodules_file | awk '{print $3}' | while read submodule_path; do
+    # Change pwd to the submodule dir and execute script
+    pushd "$gitmodules_dir/$submodule_path" > /dev/null
+    NIMBLE_DIR=$NIMBLE_DIR PWD_CMD=$PWD_CMD EXCLUDED_NIM_PACKAGES=$EXCLUDED_NIM_PACKAGES \
+    "$create_nimble_link_script_path" "$submodule_path"
+    popd  > /dev/null
+  done
+}
+
+# Create the base directory if it doesn't exist
+mkdir -p "${NIMBLE_DIR}/pkgs"
+
+# Find all .gitmodules files and process them
+for gitmodules_file in $(find . -name '.gitmodules'); do
+  echo "Processing .gitmodules file: $gitmodules_file"
+  process_gitmodules "$gitmodules_file"
+done