feat: adding idempotency to transfers (#138)

### TL;DR

Added Idempotency-Key header support to transfer endpoints.

### What changed?

- Added an optional `Idempotency-Key` header parameter to the transfer-in and transfer-out API endpoints
- The header allows clients to safely retry requests without risking duplicate transfers
- Included documentation explaining that if the same key is sent multiple times, the server will return the same response as the first request
- Added an example UUID format for the header value: `550e8400-e29b-41d4-a716-446655440000`

Author: pengying

mintlify/openapi.yaml

@@ -9,6 +9,16 @@ post:
     - Same-Currency Transfers
   security:
     - BasicAuth: []
+  parameters:
+    - name: Idempotency-Key
+      in: header
+      required: false
+      description: >
+        A unique identifier for the request. If the same key is sent multiple times,
+        the server will return the same response as the first request.
+      schema:
+        type: string
+        example: 550e8400-e29b-41d4-a716-446655440000
   requestBody:
     required: true
     content:

openapi.yaml

@@ -7,6 +7,16 @@ post:
     - Same-Currency Transfers
   security:
     - BasicAuth: []
+  parameters:
+    - name: Idempotency-Key
+      in: header
+      required: false
+      description: >
+        A unique identifier for the request. If the same key is sent multiple times,
+        the server will return the same response as the first request.
+      schema:
+        type: string
+        example: 550e8400-e29b-41d4-a716-446655440000
   requestBody:
     required: true
     content: