Materialize v0.76

v0.76.0

Allow specifying a default SSH connection when creating a Kafka connection over SSH using the SSH TUNNEL top-level option. The default connection will be used to connect to any new or unlisted brokers.
```
CREATE CONNECTION kafka_connection TO KAFKA (
    BROKER 'broker1:9092',
    SSH TUNNEL ssh_connection
);
```
Support previewing the Avro schema that will be generated for an Avro-formatted Kafka sink ahead of sink creation using the EXPLAIN { KEY | VALUE } SCHEMA syntax.

Improve connection validation for Confluent Schema Registry connections. Materialize will now attempt to connect to the identified service, rather than only building the client during validation.