[Improvement] AWS Multifactor Auth Support

Thank you @focusaurus and @gschier for your work in #347, I use it every day 😃

At the bottom of #347 you mention supporting AWS multifactor auth via the AWS_SESSION_TOKEN. If this were to be implemented I would use it every other day!

My use case: I want to get /cat/_indices and run some aws elasticsearch queries. I do this using a read only AWS_ACCESS_KEY / AWS_SECRET_ACCESS_KEY.

When I want to manipulate backups or change some settings I need write access. I generate a temporary AWS_ACCESS_KEY, AWS_SECRET_ACCESS_KEY and AWS_SESSION_TOKEN to do this.

I would love to see a third optional field that lets me put in the AWS_SESSION_TOKEN for times such as these!

[gschier]

That looks good. Nit: I think if most labels are just capitalized you could Access Key ID title case the labels. I just used the same labels that the rest of the app uses. They’re all-caps everywhere.

I wasn’t sure whether to keep the checkbox or not because only some of the auth types have it. But, I’ve decided to make it consistent and add it to all the ones that don’t as well.

[gschier]

On it! I’ll try to get this done today.