etc-merge: Implement merge

Merge added, modified, removed files from the current etc into the new
etc directory, following the rules

1. If file is removed from current_etc, it will be removed from new_etc
2. If file is modified in current_etc, it will be copied to the new_etc
   overwriting any existing files
3. If a file is added in current_etc, then the above modification rule
   applies

Modification includes change in content/permissions. Changed in Xattrs
and/or ownership is not handled yet.

Signed-off-by: Johan-Liebert1 <[email protected]>

Author: Johan-Liebert1

Cargo.lock

@@ -11,6 +11,7 @@ openssl = { workspace = true }
 hex = { workspace = true }
 tracing = { workspace = true }
 composefs = { workspace = true }
+fn-error-context = { workspace = true }
 
 [lints]
 workspace = true