Add rabbitmqadmin-ng build stage to Dockerfiles

The RabbitMQ Docker images should include the `rabbitmqadmin-ng` CLI
tool for managing RabbitMQ instances. This tool is written in Rust and
must be compiled from source with proper OpenSSL linking.

This change adds a `rabbitmqadmin-builder` stage to the Dockerfile
templates that installs Rust via `rustup-init.sh`, downloads the
`rabbitmqadmin-ng` source from GitHub, and compiles it using `cargo
build`. The build links against the existing OpenSSL installation and
configures the runtime library path via `RUSTFLAGS`.

The `versions.sh` script now fetches the latest `rabbitmqadmin-ng` release
tag from the GitHub API and calculates SHA256 checksums for both the source
archive and Rust installer. These values are stored in `versions.json` and
templated into the generated Dockerfiles.

Author: lukebakken

.gitignore

@@ -1 +1,4 @@
+*.tar.gz
+*.zip
+rustup-init.sh
 .jq-template.awk