fix(source): format upsert encode avro shall no longer fetch key schema from registry

[xiangjinwu]

After the introduction of include key as clause, we agree that kafka key will always be ingested as raw bytea and further decoding into text / json will be handled with SQL expressions.

• convert_from('\x613141'::bytea, 'utf8') -> 'a1A'::varchar
• convert_from('\x7b2279656172223a323032347d'::bytea, 'utf8')::jsonb -> '{"year": 2024}'::jsonb
• convert_from('\x313233'::bytea, 'utf8')::int8 -> 123::int8
• int8recv('\x000000000000007b'::bytea) -> 123::int8

The old logic of decoding key according to schema registry should be completely cleaned up. For now it still tries to read schema registry and reports an error when it is not found. But the schema is never used for decoding.

Note: format debezium encode avro is still reading key schema from registry.