feat: inventory items api (#2971)

What:
Admin endpoints to handle inventory items and their stock levels per location

FIXES: CORE-975

Co-authored-by: Sebastian Rindom <7554214+srindom@users.noreply.github.com>

.changeset/mean-rings-decide.md

@@ -0,0 +1,7 @@
+---
+"@medusajs/inventory": patch
+"@medusajs/medusa": minor
+"@medusajs/medusa-js": minor
+---
+
+Adding inventory items api