Mark Logan

Move is_system_tx() to TransactionData

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This will be used by accumulator settlement transactions, which will just be ordinary PTBs (with `sender = 0x0`) instead of custom transaction types

Backward compatibility layer encoding and input validation

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Implement the encoding and input validation for the backward compatibility layer Comments describing the design are here: https://github.com/MystenLabs/sui/pull/24386/files#diff-3706cd26efae5cc853c3894391c379bc8d46f621431d5b7fe6859a4bf61749b2R4

Add efficient type representations for AccumulatorMetadata and AccumulatorOwner

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The AccumulatorMetadata repr saves hundreds of bytes per balance and the AccumulatorOwner repr saves hundreds of bytes per unique balance-holding address

Mlogan bcs deser

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## Description Describe the changes or additions included in this PR. ## Test plan How did you test the new or updated feature? --- ## Release notes Check each box...

ab backward compatibility

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## Description Describe the changes or additions included in this PR. ## Test plan How did you test the new or updated feature? --- ## Release notes Check each box...