Add static stability support to IMDS credentials provider (#2258)

* Add static stability support to ImdsCredentialsProvider

This commit adds static stability support to `ImdsCredentialsProvider`.
Static stability refers to continued availability of a service in the
face of impaired dependencies. In case IMDS is not available, we still
allow requests to be dispatched with expired credentials. This, in turn,
allows the target service to makes the ultimate decision as to whether
requests sent are valid or not instead of the client SDK determining
their validity.

The way it is implemented is `ImdsCredentialsProvider` now stores a last
retrieved credentials which will later be served when IMDS is unreachable.

* Add tests to IMDS credentials provider

This commit adds tests to IMDS credentials providers for static stability
support. These tests are prescribed in https://github.com/awslabs/smithy-rs/issues/2117

.
From an IMDS credentials provider' perspective, however, some of the tests
are considered to fall under the same equivalence class with others.
Therefore, a single test can cover multiple test cases.

* Update CHANGELOG.next.toml

* Update CHANGELOG.next.toml

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

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