feat: support S3 Express One Zone

Adds some features in order to support the SDK making S3 Express requests

Issue \

Refactor checksums to use an execution context key HttpOperationContext.ChecksumAlgorithm. The value is the name of the checksum algorithm to be used. If it's set when flexible checksums runs, it will be used, otherwise the Content-MD5 header will be computed and added as before. This greatly simplifies the logic required in these interceptors.
Allow setting omitSessionToken signing configuration via execution context, using the AwsSigningAttributes. OmitSessionToken key.
Add a generic, thread-safe LruCache which holds entries up to a configurable capacity and then evicts the least-recently used entries as more are added.

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