Add GB↔GB-ZET exchange config for Shetland HVDC link

Fixes #7269 by adding the missing exchange between mainland GB and
the Shetland Islands, with 600 MW capacity from the SSEN/Ofgem link.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: sankalpsthakur

config/exchanges/GB_GB-ZET.yaml

@@ -0,0 +1,8 @@
+_comment: 600MW Shetland HVDC Link (SSEN Transmission) https://www.ofgem.gov.uk/sites/default/files/2021-11/Shetland%20Link%20Project%20Assessment%20Decision%20FINAL.pdf
+capacity:
+  - -600
+  - 600
+lonlat:
+  - -2.2
+  - 59.35
+rotation: 45