cmake: allow corrosion to be provided externally

the default behavior (retrieving Corrosion from
github:corrosion-rs/corrosion) is not changed, but this change allows
vcpkg or Nix to supply Corrosion instead.

Author: lbeierlieb

vmicore/rust_src/CMakeLists.txt

@@ -1,10 +1,13 @@
-include(FetchContent)
-FetchContent_Declare(
-        Corrosion
-        GIT_REPOSITORY https://github.com/corrosion-rs/corrosion.git
-        GIT_TAG stable/v0.4  # Newer versions require cmake 3.22 which is not available on Debian 11
-)
-FetchContent_MakeAvailable(Corrosion)
+find_package(Corrosion CONFIG)
+if (NOT Corrosion_FOUND)
+    include(FetchContent)
+    FetchContent_Declare(
+            Corrosion
+            GIT_REPOSITORY https://github.com/corrosion-rs/corrosion.git
+            GIT_TAG stable/v0.4  # Newer versions require cmake 3.22 which is not available on Debian 11
+    )
+    FetchContent_MakeAvailable(Corrosion)
+endif()
 
 corrosion_import_crate(MANIFEST_PATH Cargo.toml)
 if (${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang")