Add jitter to `LazyCredentialsCache` (#2335)

* Add jitter to `LazyCredentialsCache`

This commit adds jitter to `LazyCredentialsCache`. A jitter provides a
mechanism for randomizing the buffer time for credentials. This allows
credentials with the same expiry to expire at slightly different times,
thereby preventing thundering herds.

* Update CHANGELOG.next.toml

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