feat(aws-config): Add IMDSv2 Credentials Provider (#709)

* feat(aws-config): Add IMDSv2 Credentials Provider

This follows up on #701 to use IMDS to act as a credentials provider. IMDS sits in the chain in two places:

1. IMDS may be used as a `credential_source` in `~/.aws/config`
2. IMDS is the last provider in the default chain of credential providers.

The IMDS credential provider may also be used directly.

* fix doc links

* Update aws/rust-runtime/aws-config/src/imds/credentials.rs

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* Update aws/rust-runtime/aws-config/src/imds/credentials.rs

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* CR feedback

Co-authored-by: John DiSanti <jdisanti@amazon.com>