Add support for Snowflake Key Pair Authentication

[alexyer]

This PR adds support for Snowflake Key Pair Authentication.

Unit tests verify everything's getting passed through correctly.

[drewbanin]

Thanks for the PR @alexyer! I love this feature, but the cryptography.hazmat imports are giving me some pause. It looks like Snowflake recommends doing it this way, but I wonder if there isn't a higher-level approach that we could take here. @beckjake do you have any thoughts on this?

[beckjake]

hazmat is where the cryptography library stores all the crytpo primitives people actually use because they want you to use higher-level constructs. This is just encoding/serialization so there is no higher level construct to use.

I don't think I've ever accomplished a single useful thing from that library without calling into hazmat. This looks good to me.

[drewbanin]

@alexyer I gave this a spin locally and it works great! I'll kick off the full test suite and then I think we'll be good to merge :)

[alexyer]

@drewbanin great, thanks for the review!

[drewbanin]

Looks like the tests failed on the unittest.mock import. dbt is currently tested on python 2 & 3, and I think unittest.mock is exclusive to py3

[alexyer]

Oops, my bad. I'll update my PR.

[drewbanin]

hey @alexyer - I just kicked off another build for this, and it looks like there's a minor error with the unit tests. I'd love to get this merged, let me know how I can help!

[alexyer]

@drewbanin just a minor typo. Should be fixed now.

[drewbanin]

Thanks @alexyer - merged :)