Exchange client credentials for an access token.

The returned token includes scopes that determine API access:

• shipper scope grants access to /v1/shipper/** endpoints
• carrier scope grants access to /v1/carrier/** endpoints
• Partners with both scopes can access both APIs with a single token

The returned token should be included in subsequent API requests as a Bearer token
in the authorization header: authorization: Bearer {access_token}

Tokens are valid for the duration specified in expires_in (seconds).
We recommend caching tokens and refreshing them before expiration.