Update AWS auth method certificates
Add tests that the `rsa2048` document can also be verified using the
`pkcs7` field for AWS auth.
Due to the use of SHA-1-based signatures for the `identity` and `pkcs7`
methods, we want to encourage moving toward using the RSA 2048 workflow,
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/verify-rsa2048.html
This doesn't require code changes for Vault necessarily, but adding in
the (many) certificates will help end users.
Also adds `rsa2048` option to API to fetch the RSA 2048 signature.
I will make a PR to update to the AWS auth docs to document the RSA 2048
flow soon after this.
Christopher Swenson