fix(dashboard,medusa): Fixes to Customer and Customer Groups domains (#7081)

**What**
- Cleanup of domains
- Adds toasts
- Adds delete customer hook
- Fixes validation of create and update customer endpoints.

.changeset/young-geckos-battle.md

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+---
+"@medusajs/medusa": patch
+---
+
+fix(medusa): Fix validation of V2 POST /customers and POST /customers/:id