add snowflake external oauth sample

[zsun-tableau]

Things To Remember:

1. Be sure to merge into the right branch. The choices are:

• master if your PR runs on the current version of Tableau.
• dev-2021.2

2. You should update at least one of the following:

• The changelog of the project you modified.
• The root README.md if the PR is a release of the SDK, Connector Packager, or TDVT.
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[salesforce-cla]

Thanks for the contribution! Unfortunately we can't verify the commit author(s): Alex(Zhaoyu) Sun <z***@s***.com>. One possible solution is to add that email to your GitHub account. Alternatively you can change your commits to another email and force push the change. After getting your commits associated with your GitHub account, refresh the status of this Pull Request.