Add new option oauth_cb to SchemaRegistryClient

[bodepd]

This patch adds an option to supply an oauth callback to the SchemaRegistryClient so that it accepts oauth tokens in the same way that kafka brokers can receive tokens.

The function passes has the same return types as the same option for kafka so that users can reusing their existing token service for both kafka as well as the schema registry client.

If a user supplies this new option together with BASIC auth configuration, it results in a ValueError.

[bodepd]

I noticed some of the other patches have a CLA notification. Is there a CLA that I need to sign?

[cla-assistant[bot]]

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