fix: parse variable scale decimal in debezium json

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What's changed and what's your intention?

#10628

we cannot handle fixed scale and precision decimal because we don't see debezium schema when parsing.

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Release note

[xiangjinwu]

We shall decode the JSON string as base64 to get bytes, rather than getting the UTF-8 bytes of this JSON string. Example in #10628 (comment):

• JSON: "price":{"scale":1,"value":"AM0="}
• base64 decode result: [0x00, 0xcd]
• as_bytes: [0x41, 0x4d, 0x30, 0x3d]

Let me fix it.