IMDS Client (#701)

* feat(aws-config): Add IMDS Client

* Fix docs

* Add two missing error implementations + usage example

* Fix clippy errors

* Fix doctest in os_shim_internal

* Fix a missed occurence of test connection assertions

* fix doc link

* remove link to private item

* Update changelog, make depedencies optional

* CR feedback cleanups

* Apply suggestions from code review

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

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

* refactor cache into its own module

* fix clippy warning

* add additional internal changelog entries

* endpoint mode comparsion should be case insensitive

* update docs to remove references to credentials

* Fix bug in token that doubled the buffer period

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