KIP-1054 — Support External schema in JSONConvertor

Accepted Kafka 4.2 Connect

Introduces an `external.schema` configuration option for `JsonConverter` in Kafka Connect that reads the schema from a file path rather than embedding it in every message payload. When connectors require a schema (e.g., JDBC connectors) but the message producer cannot embed a schema in each JSON record, the current approach either bloats every message with a full schema copy or requires a schema registry.

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