Add user-agent 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.

Issue - https://github.com/Strech/avrora/issues/101

Reference:

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

[Strech]

Hey @azeemchauhan

Thanks for opening PR I appreciate it. I will take a look and possible push some adjustments alongside yours in this PR.

[Strech]

@azeemchauhan Thanks for your support. A new version 0.25.0 was just released with your changes.