Remove `express_cache` feature flag

[vladem]

Description of change

• Removes the feature flag so the shared cache may be included in the next build;
• Adds a changelog entry introducing the feature.

(update and merge this after: https://github.com/awslabs/mountpoint-s3/pull/1144)

Relevant issues: N/A

Does this change impact existing behavior?

No, a new feature added.

Does this change need a changelog entry in any of the crates?

Yes, adding one in this PR.

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