Add Rust-based rabbitmqadmin-ng with Python fallback

The RabbitMQ management images now include the Rust-based
`rabbitmqadmin-ng` CLI tool when possible, with automatic fallback to
the Python version on architectures where Rust compilation fails.

This change adds a two-stage build to `Dockerfile-management.template`:
- **Builder stage**: Attempts to compile `rabbitmqadmin-ng` from source
  using Rust toolchain installed via `rustup-init.sh`. The build links
  against the custom OpenSSL installation using `OPENSSL_DIR` and
  `RUSTFLAGS` with proper rpath configuration.
- **Final stage**: Copies the Rust binary if build succeeded, otherwise
  falls back to the Python script from the RabbitMQ management plugin.
  Python is only installed when using the fallback.

The `versions.sh` script 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 management Dockerfiles.

This approach ensures smaller images on architectures with good Rust
support (amd64, arm64v8) while maintaining compatibility with all
supported architectures through automatic Python fallback.

Author: lukebakken

.gitignore

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