KIP-421 — Support resolving externalized secrets in AbstractConfig

Accepted Kafka 2.3 ConnectAdmin

Adds a ConfigProvider SPI to Kafka's config system that allows config values to be resolved from external sources (Vault, environment variables, files) at startup using a ${provider:key} syntax in config files. Secrets like passwords and credentials are currently stored in plaintext in server.properties and connector configs, with no standard mechanism to retrieve them from secret managers.

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