KIP-437 — Custom replacement for MaskField SMT

Accepted Kafka 2.6 Connect

Extends the `MaskField` SMT to accept a configurable `replacement` value so masked fields can be replaced with a custom string or number (e.g. `***-***-****`) instead of the type's null equivalent. The original `MaskField` only replaced values with `null` or zero, making it unusable for PII redaction scenarios that require a recognizable sentinel value rather than nulling the field.

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