Integration for GrubCC and SystemdBoot

[Johan-Liebert1]

Add Bootloader enum

Now that we are going to support multiple bootloaders, we need a way to
keep track of which one we're operating on

---

For bootupd generate-update-metadata we now generate metadata for all
found bootloaders. The final json will look like

{
  "timestamp": "2026-06-10T09:52:58.107743026Z",
  "version": "grub-cc-1:2.12-59.fc45,grub2-1:2.12-58.fc44,shim-16.1-5",
  "versions": [
    { "name": "grub-cc", "rpm_evr": "1:2.12-59.fc45" },
    { "name": "grub2", "rpm_evr": "1:2.12-58.fc44" },
    { "name": "shim", "rpm_evr": "16.1-5" }
  ]
}

This allows us to have multiple bootloaders in a single image and
install/update based on preference.

During install/update, we check to bootloader and filter out any and all
entries from the update metadata that do not match or are not required
for the bootloader

---

For grub we store the statefile in /sysroot/boot/bootupd.json. For
grub-cc, and in future systemd-boot, we won't have a /boot, so we now
store the state pre bootloader.

Grub2: Statefile is still in /sysroot/boot/bootupd.json
GrubCC: Statefile is stored in all the ESPs

Store the sysroot_path in StateLockGuard which we require for mounting
the ESP. Also, mount the ESP at /boot which is what the BLS spec suggest

---

[gemini-code-assist]

Code Review

This pull request introduces support for multiple bootloaders, specifically adding a new GrubCC option alongside the default Grub. It updates the saved state, component installation, update, and status mechanisms to dynamically detect and filter metadata based on the active bootloader, including handling state files stored in the ESP for GrubCC. The review feedback identifies a potential mount leak where the was_mounted flag is not reset upon unmounting, several instances of unnecessary double referencing, and opportunities to make the code more idiomatic by simplifying OnceLock usage and removing redundant iterator adapters.

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