Implement an identity cache in aws-smithy-runtime (#3062)

This PR adds a `ResolveCachedIdentity` trait, and adapts the existing
LazyCredentialsCache implementation to it. It does NOT make this cache
used yet, as that will be done as a separate PR to keep code review
smaller.

Notable differences from the credentials cache:
- Supports multiple different identity types and identity resolvers
- Applies cache partitioning to the `SharedIdentityResolver` so that
operation config overrides of the identity resolver will result in the
correct identity being resolved.

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