Empty user-agent in header when calling registry

[azeemchauhan]

Background
AWS WAF's core managed rule group contains a rule that rejects requests missing a user-agent header. If the schema registry on AWS behind a WAF with this rule group, which blocks all requests. To avoid this, we should set a user agent in the HTTP header.

Reference:

• Use AWS Managed Rules to prevent common attacks
• kafkajs/confluent-schema-registry