Allow configuring basicAuth for avro schema registry

[d-rk]

Description

Please include a summary of the change and which issue is fixed.
List any dependencies that are required for this change.

Fixes #208

Type of change

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• New feature (non-breaking change which adds functionality)

Documentation

• the change is mentioned in the ## [Unreleased] section of CHANGELOG.md
• the configuration yaml was changed and the example config in README.adoc was updated
• a usage example was added to README.adoc
• tests for the changes have been implemented (see: Testing your changes)