Add missing API operations to Schema Registry client

[alexhein]

1. Add the API path to list contexts: https://docs.confluent.io/platform/current/schema-registry/develop/api.html#get--contexts
2. Add the API path to retrieve all subject-version pairs associated with a given schema ID: https://docs.confluent.io/platform/current/schema-registry/develop/api.html#get--schemas-ids-int-%20id-versions

[cla-assistant]

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[alexhein]

@rayokota Can you please take a look?