Support for IMS service token generation plugin

```
curl --location --request POST 'https://ims-na1-stg1.adobelogin.com/ims/token/v1' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--form 'grant_type=authorization_code' \
--form 'client_id=' \
--form 'client_secret=' \
--form 'scope= \
--form 'code='
```

The issue names no files or tests. Use the supplied IMS token-generation curl request as the behavioral reference, then inspect the library's existing authentication and token entry points. Done means the JavaScript SDK supports generating an IMS service token with the shown grant type and fields.

Written by the indexing model from the issue text.