Add CAT param to b2b auth storefront endpoint (#698)

<!-- Ticket number or summary of work -->
# NO TICKET


## What changed?
Add a new parameter CustomerAccessToken in the b2b
io/auth/customers/storefront endpoint.

## Release notes draft
Enhancement: Added an optional parameter to the storefront
authentication endpoint for b2b.

<!-- Provide an entry for the release notes using simple, conversational
language. Don't be too technical. Explain how the change will benefit
the merchant and link to the feature.


## Anything else?
NA

ping {names}

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

@@ -591,13 +591,17 @@ paths:
                 channelId:
                   type: integer
                   description: BigCommerce channel ID
+                customerAccessToken:
+                  type: string
+                  description: BigCommerce Customer Access Token
               required:
                 - customerId
             examples:
               Example 1:
                 value:
                   customerId: 999
                   channelId: 1
+                  customerAccessToken: <sometoken>
       tags:
         - Auth
 components: