feature: support kafka schema registry for Protobuf and Avro.

[zliang-min]

PR checklist:

• Did you run ClangFormat ?
• Did you separate headers to a different section in existing community code base ?
• Did you surround proton: starts/ends for new code in existing community code base ?

Please write user-readable short description of the changes:

New settings added for using kafka schema registry: kafka_schema_registry_url and kafka_schema_registry_credentials. Example:

CREATE EXTERNAL STREAM my_stream (
  -- columns goes here ...
) SETTINGS
type = 'kafka',
brokers = '...',
topic = '...',
data_format = 'ProtobufSingle',
kafka_schema_registry_url = 'http://url.to/my/schema/registry',
kafka_schema_registry_credentials = '$api_key}:${secret}';

kafka_schema_registry_credentials is optional, it contains the the API key and secret and a : in between. Also, kafka schema registry can be used with data_format = 'Avro'.

Note: in this PR, schema reference is not supported yet.

[chenziliang]

In general, it looks good and let's clean up the old code which is commented out