`schema-registry` feature (baseline)

[jfallows]

Baseline

• Design zilla.yaml configuration syntax for schema types #310
• Enforce inbound type checking; kafka cache produce client #312
• Apply typed schema to cached messages; kafka cache fetch server #314
• Integrate Schema Registry / Karapace #404
• Support inline catalog for validators #453
• Implement json validator #454
• Integrate mqtt binding with validators #456
• Integrate http binding with validators #455