Support comma-separated list of schema registry URLs

[morrone]

Description

It would be nice if the SchemaRegistryClient() support a "url" string which is a comma-separated list of schema registry URLs instead of just a single schema registry URL. I believe that libserdes supports a comma-separated list in schema.registry.url (see libserdes README).

How to reproduce

Try putting a comma-separated string of URLs as the value of the "url" configuration option.

Checklist

Please provide the following information:

• confluent-kafka-python and librdkafka version (confluent_kafka.version() and confluent_kafka.libversion()):
• Apache Kafka broker version:
• Client configuration: {...}
• Operating system:
• Provide client logs (with 'debug': '..' as necessary)
• Provide broker log excerpts
• Critical issue