Add cross configuration

This allows running unit tests on a non-linux platform via the `cross test` command.

Author: astapleton

Cross.toml

@@ -0,0 +1,13 @@
+# Cross doesn't seem to look for package specific configurations when invoked in a workspace so
+# we put the default target in Cross.toml
+
+[build]
+default-target = "aarch64-unknown-linux-gnu"
+
+# Allow usage of locally built version of the cross docker image that matches the host architecture
+# e.g. Building a linux/arm64 image for an AARCH64 host allows native execution of the image.
+# Specifying the toolchain for a linux/arm64 image means the correct toolchain is used on that
+# platform (it defaults to x86_64).
+[target.aarch64-unknown-linux-gnu.image]
+name = "ghcr.io/cross-rs/aarch64-unknown-linux-gnu:main"
+toolchain = ["linux/arm64=aarch64-unknown-linux-gnu"]