Refactor for async support

[rohitsanj]

What

This PR refactors the schema registry modules across a newly introduced _sync package, as well as the common package. The PR that introduces the _async package is stacked onto this one, see #1965.

To illustrate the changes made, let's take src/confluent_kafka/schema_registry/schema_registry_client.py for example:

• Some of the common functionality was refactored out into src/confluent_kafka/schema_registry/common/schema_registry_client.py -- this package is intended to house functionality that is common to both the sync and async implementations.
• The sync implementations of SchemaRegistryClient and its dependent classes were moved into src/confluent_kafka/schema_registry/_sync/schema_registry_client.py
• The original src/schema_registry/schema_registry_client.py was refactored to only contain imports from the common.schema_registry_client and _sync.schema_registry_client modules. This is done so that users relying on imports directly from confluent_kafka.schema_registry.schema_registry_client don't encounter any breaking/missing imports.

The above treatment was applied to: schema_registry_client.py (as demonstrated), avro.py, json_schema.py, protobuf.py and serde.py.

Other changes:

• The corresponding test files have also been moved into a new subpackage called _sync (with minimal changes).

Checklist

• Contains customer facing changes? Including API/behavior changes
• Did you add sufficient unit test and/or integration test coverage for this PR?
  • If not, please explain why it is not required: PR only contains refactorings, no functionality has been added or removed.

References

JIRA:

Test & Review

Open questions / Follow-ups

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[sonarqube-confluent]

• 72.50% Coverage on New Code (is less than 80.00%)

Analysis Details

46 Issues

• 0 Bugs
• 1 Vulnerability
• 45 Code Smells

Coverage and Duplications

• 72.50% Coverage (62.50% Estimated after merge)
• No duplication information (1.00% Estimated after merge)

Project ID: confluent-kafka-python

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[rohitsanj]

unittest.mock.patch needs to be used directly with the module that defines the function to be patched, hence this change.