Add constructor for JsonDecoder in AvroCompatibilityHelper

Currently, there is no constructor for JsonDecoder that is available in AvroCompatibilityHelper which accepts (Symbol, InputStream) as arguments.
org.apache.avro.io.parsing.Symbol is in all avro versions so adding it will help out MPs that require this for their Avro migration.

Start by locating AvroCompatibilityHelper and its existing JsonDecoder constructors or factory methods. Add support for the (Symbol, InputStream) arguments using the Avro-compatible APIs described in the issue, and verify that callers can construct a JsonDecoder with those arguments across supported Avro versions.

Written by the indexing model from the issue text.