Support for SSL for the connection to Schema Registry is needed

Support for SSL certificates and keystores configuration options for the connection to Schema Registry is needed.

Currently, this is not possible to connect to Schema Registry, requiring client SSL certificate, with `libserdes`:
```
$ ./kafkacat -b $KAFKA_BROKER_HOST:$KAFKA_BROKER_PORT \
> -t my-topic \
> -s value=avro \
> -r https://$KAFKA_API_KEY:$KAFKA_API_SECRET@$KAFKA_SCHEMA_REGISTRY_HOST:$KAFKA_SCHEMA_REGISTRY_PORT \
> -X security.protocol=SASL_SSL \
> -X sasl.mechanisms=PLAIN \
> -X sasl.username="$KAFKA_API_KEY" -X sasl.password="$KAFKA_API_SECRET" \
> -X ssl.keystore.location=$KAFKA_KEYSTORE_P12_LOCATION \
> -X ssl.keystore.password=$KAFKA_KEYSTORE_PASSPHRASE \
> -X ssl.key.password=$KAFKA_KEY_PASSPHRASE \
> -X enable.ssl.certificate.verification=false \
> -o beginning -C
% ERROR: Failed to format message in my-topic [0] at offset 2: Avro/Schema-registry message deserialization: REST request failed (code -1): HTTP request failed: SSL connect error : terminating
```

Start by locating the libserdes Schema Registry REST connection code and its existing configuration options. Use the kafkacat command and SSL keystore settings in the report as the reproduction case; done means a client certificate and keystore can be configured for a successful Schema Registry connection.

Written by the indexing model from the issue text.