uefi: Add device path generation for discovered devices in a PciTree

[seijikun]

Initial implementation for device path generation of discovered pci devices in a PciTree instance.
Unfortunately, it's a bit clunky to use, since you have to query the DevicePath corresponding to your PciRootBridgeIo instance yourself and pass that in:

let mut pci_proto = get_open_protocol::<PciRootBridgeIo>(pci_handle);
let root_device_path = get_open_protocol::<DevicePath>(pci_handle);

let pci_tree = pci_proto.enumerate().unwrap();
for addr in pci_tree {
    // v Would be nicer, if I didn't have to pass root DevicePath in manually here v
    let device_path = pci_tree.device_path(&root_device_path, addr).unwrap();
}

I tried getting the DevicePath of the PciRootBridgeIo instance within PciRootBridgeIo::enumerate() and pass that on to the PciTree, but I couldn't find a way to get the DevicePath instance - other than ugly workarounds.

Best consumed Commit-By-Commit. Contains #1830..

Steps to Undraft

• merge and rebase onto uefi: Refactor PciRootBridgeIo::enumerate() with tree-topology information #1830

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[phip1611]

LGTM with two nits!

PS: You do not have to fix it here but do you have an opinion on #1817? I'd like to hear a direction that we should go for (Box<DevicePath> vs PoolDevicePath)

[phip1611]

Correct me if I'm wrong but I think #[cfg(feature = "alloc")] is not needed here. PoolDevicePath works with the underlying UEFI allocator and does not interfer with the global Rust allocator at all?

The feature alloc is only for the global Rust allocator and not all allocations in general.

[phip1611]

this should implement core::error::Error and should also implement fn source() properly.