add pem option to doc

[jschenz-chwy]

What are you changing in this pull request and why?

This change describes the option of using a plain-text PEM format private key for authentication with Snowflake added in dbt-labs/dbt-snowflake#671.

Checklist

• Review the Content style guide and About versioning so my content adheres to these guidelines.
• Add a checklist item for anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."

Checklist:

• The PR fix: add option to use private key for authentication dbt-snowflake#671 needs to be merged
• This doc will need to be updated with the dbt-snowflake version that supports the PEM key option.

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[mirnawong1]

hey @jschenz-chwy thanks for opening this pr up! Since this is for v1.7, we will go ahead and merge this for you then (3 months from now) ✨

not ready for merge yet

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