fix(avro): parse decimals with shorter bytes correctly

[xiangjinwu]

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What's changed and what's your intention?

See risingwavelabs/avro#3. In short, given decimal(10, 2) it would need 5 bytes to store its maximum. When decoding an actual decimal with 4 or few bytes (e.g. simply 0 or 42), the upstream library reports an error (wrong comparison due to misconception similar to the missing dollar riddle).

Actual fix is in the forked library, while this PR just bumps the commit and adds a test. To test decimal (a logical type over bytes in avro), we had to update the test input format: it used to be plain json but switches to the json encoding (contrasting to binary encoding in kafka) of avro. The first commit of this PR is this pure test-refactor but not a standalone PR to ease cherry-picking into release branch.

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