[Credential] Support calls to IMDSv2 when retrieving instance credentials

[richarddotcodes]

I recently ran into an issue when enforcing v2 calls to the EC2 Instance Metadata service, as Rusoto appears to only use v1 (no token) calls: https://docs.rs/rusoto_credential/latest/src/rusoto_credential/instance_metadata.rs.html#117

I'd like to submit a PR that attempts to retrieve a token before making the call, and falls back to a v1 call if a token can't be obtained. Any thoughts on this?

[wimax-grapl]

#1818 (comment)

[wimax-grapl]

tldr yes please!