Allow user to be omitted if using authenticator

[MasterOdin]

resolves #726
docs dbt-labs/docs.getdbt.com/#

Problem

Currently, when using authenticator: oauth, you have to pass it client id & secret & token which is enough to authenticate, but also need to still pass user (which is validated if it has a value).

Solution

This PR makes user optional so that it can be omitted when using an authenticator, otherwise an error will be raised.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

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