Issues an OAuth 2.0 access token using the Client Credentials grant flow.

The client authenticates using HTTP Basic authentication — the Authorization header
must contain the Base64-encoded client_id:client_secret pair:

Authorization: Basic Base64(client_id:client_secret)

The request body must be sent as application/x-www-form-urlencoded.

How to use the token

Once issued, include the access_token value in every subsequent API call as a
Bearer token in the Authorization header:

Authorization: Bearer {access_token}

The token is valid for the number of seconds indicated by expires_in. After expiry,
request a new token using this endpoint.