Add user agent header to the schema registry rest request

[glvecino]

Description

The schema registry RestService implementation: https://github.com/confluentinc/confluent-kafka-dotnet/blob/master/src/Confluent.SchemaRegistry/Rest/RestService.cs does not allow any mechanism to provide headers.
In our company we have a WAF blocking any request without UserAgent, so we would like to be able to add at least that header.

How to reproduce

Just in any call of the schema registry captured the UserAgent header is not present.

Checklist

• Confluent.SchemaRegistry --version 2.3.0