Adds teleport support

[userhas404d]

Hey @jmkeyes thanks for building and maintaining this project!

Disclaimer: I don't work for gravitational but my org uses teleport heavily

These changes are a result of not being able to easily leverage the teleport client as a credential process via aws config (more details on how this client works available here). I realize not many folks are going to have a use case for this so if you'd prefer that this not get merged I completely understand. I tried to make a point to avoid any major changes to the existing client config and maintained boto3 as the default. Fully testing this without a working teleport implementation is also going to be a challenge but hopefully with it being an optional client config that's not a huge deal.

[jmkeyes]

Hey @userhas404d, thanks for submitting this PR! I believe this keyring backend should already transparently support using Teleport.

I read through the Teleport documentation and found that it can launch a local proxy gateway to communicate with AWS for applications using the AWS SDK:

$ tsh proxy aws -p 23456
Started AWS proxy on http://127.0.0.1:23456.

Use the following credentials and HTTPS proxy setting to connect to the proxy:
  export AWS_ACCESS_KEY_ID=abcd1234-this-is-an-example
  export AWS_SECRET_ACCESS_KEY=zyxw9876-this-is-an-example
  export AWS_CA_BUNDLE=<ca-bundle-path>
  export HTTPS_PROXY=http://127.0.0.1:23456

The AWS SDK (ie: boto3) should react to the presence of those environment variables and use them automatically, so long as they are available to the process (ie: pip / twine) doing the keyring lookup.

Have you encountered issues using the proxy with this library? I would try this myself, but I don't have a Teleport account to test it out.