Switch from bash to rust

[TobiPeterG]

This PR aims to switch from the bash script to a rust based approach. Currently, all functions available in sdbootutil are callable and have their designed parameters. A help page exists and is auto-generated. However, the functions still lack the specific implementation, there currently is no testing framework in place and no github actions are used to e.g. build the source, test for warnings and execute tests.

Contributions are welcome :)

EDIT: Github Actions are added, will automatically build and run the specified tests

Basic tests are added

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Implemented functions

• [x] help
• [ ] kernels
• [ ] snapshots
• [ ] entries
• [x] bootloader
• [ ] add-kernel
• [ ] all-all-kernels
• [ ] mkinitrd
• [ ] remove-kernel
• [ ] remove-all-kernels
• [ ] list-kernels
• [ ] list-entries
• [ ] list-snapshots
• [ ] set-default-snapshot
• [ ] is-bootable
• [x] is-installed
• [x] install
• [x] needs-update
• [x] update
• [x] force-update
• [ ] update-predictions

Implemented functionality

• [x] Basic UI
• [ ] UI Main menu
• [ ] UI Kernels menu
• [ ] UI Entries menu
• [ ] UI Snapshots menu
• [ ] UI install/Update menu
• [x] README with contribution instructions
• [x] command line parsing

Git hates me. See discussion so far here: #75