Handle slash in subject name while performing Schema Registry API calls

[saiharshavellanki]

Description

• One of the limitations mentioned in schema registry documentation is that subject name can contain "/" character. However, we have to escape it in the schema registry APIs
• This change replaces "/" in the subject name with "%2F" thereby allowing API calls to succeed

Use case

• We have one of our schemas referencing other schema containing "/" in its name. Due to the above mentioned limitation, schema_registry_converter::async_impl::proto_decoder::deserialize_with_context method is returning Could not get id from response as the response. After this fix, deserialize_with_context is returning valid data and things are working E2E as expected

Testing

• Tested on my confluent schema registry with subject name as "a/b/c.proto" API call failing with {"error_code":404,"message":"HTTP 404 Not Found"} error before the change and the call has succeeded after changing subject name to "a%2Fb%2Fc.proto"