include the userid in the dashboard footer

[martinm-nm]

Context

At the moment libddog appends a footer in the dashboard description which includes the timestamp and the git branch name (if detected) of the last update to the dashboard. This already helps to narrow down how that last update happened, but it would be even more useful to also include the userid of the user who made the update (or whose credentials were used to make the update).

It looks like the user identity can be inferred from the app key via the Datadog API:

1. Make a request to https://api.datadoghq.com/api/v2/current_user/application_keys to list all the app keys for the current user. Iterate over the list and pick the key that matches the app key that's being used in the session.
2. Make a request to https://api.datadoghq.com/api/v2/current_user/application_keys/<app-key-id> to get details about that app key. In the response there is a handle attribute which contains the userid.

[martinm-nm]

Going by the API docs it looks like this would require migrating to the datadog-api-client package.