DEVDOCS-6590 - Add customerId to get super admin details response (#1148)

<!-- Ticket number or summary of work -->
# [DEVDOCS-6590]


## What changed?
We are now adding the customerId field to the response of this endpoint
so that developers do not need to call multiple APIs to identify a Super
Admin by their email address.

## Release notes draft
Added `customerId` field to the Get Super Admin Details v3 api endpoint

ping @bc-terra 


[DEVDOCS-6590]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6590?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: chris-nowicki

docs/b2b-edition/specs/api-v3/super_admin/super_admin.yaml

@@ -412,6 +412,10 @@ paths:
                         allOf:
                           - $ref: "#/components/schemas/superAdminResponse"
                           - properties:
+                              customerId:
+                                type: integer
+                                description: 'Customer ID of the Super Admin account.'
+                                example: 343
                               extraFields:
                                 $ref: "#/components/schemas/extraFieldsArray"
         '404':