Backends api

[tristanpoland]

This pull request adds a new backend_api and updates the build process to handle backend and plugin discovery. The most important changes are the addition of a new backend_api package, updates to the Cargo.toml files, and enhancements to the build script to automate backend and plugin management.

New Package Addition:

• Added a new backend_api package with its own Cargo.toml file, including dependencies and build dependencies.

Updates to Cargo.toml:

• Modified the main Cargo.toml to include the new backend_api package and removed the chronos_plugin.

Build Script Enhancements:

• Added a new build.rs script in the backend_api package to automate the discovery of backends and plugins, update Cargo.toml files, and generate necessary backend files.# This PR adds the .allow-imports file flag

The adding the .allow-imports file to your backend root allows backends to import plugins. Adding this file to a plugin allows it access to other plugins (Adding this flag to plugins is NOT recommended as it creates some error prone behavior due to missing dependencies).
The Horizon plugins and backends APIs respectively will handle this automatically pre-compile-time to allow for inter plugin and backend-to-plugin communication.

Notes

• You will not be able to use other plugins with .allow-imports enabled in another .allow-imports plugin, this is to prevent circular dependencies entirely.
• This is an unstable feature, it may change in the future until it's announced as stable.

[tristanpoland]

I have yet again marked this PR as a draft as I am rethinking other possible ways to execute this. It is possible that we will go for it an entirely separate dedicated back-end API as opposed to nearly hooking into the existing plugins API. This will help keep plug in life cycle management to clean as well as keep the plug-in spec clear and concise.

[tristanpoland]

We are finally ready to merge this. The dedicated backends API is complete! 🎉

[WilliamAnimate]

looks good