Add CalDAV support, refactor

The filesystem storage backend now implements the required functions to
act as a basic CalDAV server. Some refactoring was done based on the
go-webdav development: introduce a UserPrincipalBackend, a new function
to serve the user principal URL, and more. See this PR for lots of
details: https://github.com/emersion/go-webdav/pull/62

Also adds a simple facility for debug output.

debug/debug.go

@@ -0,0 +1,15 @@
+package debug
+
+import (
+	"log"
+)
+
+var (
+	Enable = false
+)
+
+func Printf(format string, v ...any) {
+	if Enable {
+		log.Printf("[debug] "+format, v...)
+	}
+}