fastn_ds Fix: fastn_ds::Path strip_prefix function

[Arpita-Jaiswal]

This PR has following changes:

Change the signature of fastn_ds::Path::strip_prefix function from:

pub fn strip_prefix(&self, base: &Self) -> fastn_ds::Path

to

pub fn strip_prefix(&self, base: &Self) -> Option<fastn_ds::Path> 

Move fastn_ds::Path::exists to fastn_ds::DocumentStore::exists

- impl fastn_ds::Path {
-    pub fn exists(&self) -> bool {
-       self.path.exists()
-    }
- }

+ impl fastn_ds::DocumentStore {
+    pub fn exists(&self, path: &fastn_ds::Path) -> bool {
+        path.path.exists()
+    }
+ }