KIP-632 — Add DirectoryConfigProvider

Accepted Admin

Adds a `DirectoryConfigProvider` that reads configuration values from files in a directory (one file per key, filename = key name) rather than from a properties file. On Kubernetes, Secrets are mounted as a directory with one file per key; the existing `FileConfigProvider` (KIP-297) reads only `.properties` files, making it ergonomically incompatible with the Kubernetes Secret directory mount pattern.

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