refactor(source): ParserProperties::key_encoding_config is introduced but never used

[xiangjinwu]

It was introduced as a part of the new source DDL work, aiming to separate key and value properties. #10969

However, for backward compatibility, we still need to support reading key properties from the unified properties. So the refactor was paused.

pub struct SpecificParserConfig {
    pub key_encoding_config: Option<EncodingProperties>, // newly introduced but never used
    pub encoding_config: EncodingProperties, // unified key+value, would like to be repurposed as value-only
    pub protocol_config: ProtocolProperties,
}

The current status is misleading / confusing. We need to decide whether to revert or continue the separation.