Improve logging for the credentials cache (#3428)

## Motivation and Context
Make it easier to understand why credentials are expired. New log
example:

```
2024-02-19T17:47:31.769924Z  INFO lazy_load_identity: aws_smithy_runtime::client::identity::cache::lazy: identity cache miss occurred; added new identity (took 861.893ms) new_expiration=2024-02-19T18:47:31Z valid_for=3599.230108s partition=IdentityCachePartition(1)
```

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._