feat(eficore): find the extended boot loader partition on the disk of a partition

Look for a filesystem on the same disk whose GPT partition type is the
XBOOTLDR type, using the partition info protocol. open_shared moves to
the handle module so the graphical menu and the discovery share it.

crates/eficore/src/lib.rs

@@ -27,6 +27,9 @@ pub mod platform;
 /// Secure Boot support.
 pub mod secure;
 
+/// Finding the Extended Boot Loader Partition.
+pub mod xbootldr;
+
 /// Support for the shim loader application that enables Secure Boot.
 pub mod shim;