Client id on all Requests for tracking

[chrisoti]

We are currently using your module and with the help of Salesforce's Administrator Reports we are currently seeing last week's API Calls.
The issue is the client_id 'simple-salesforce' or 'simple-salesforce/custom-client-id' is only used and applied in login requests.

We would like if its possible and you have the time to add the same feature in the rest of the requests as well.

It will be a really good feature to be able to know the source of the API Call so we can know which integration needs optimization and what is its impact on the API Limit Usage that Salesforce has on each integration function.