Add initial Transaction V6 support to Zebra (currently copies V5 behavior)

[dmidem]

This PR introduces initial support for Transaction version 6 (Tx V6) in the Zebra codebase as part of the broader effort to add ZSA (Zcash Shielded Assets) support.

Key changes

• Added a new V6 variant to the Transaction enum in the zebra-chain crate, currently replicating the behavior of V5.
• Updated relevant code paths across the project to handle the new V6 variant, ensuring basic compatibility.
• Tx V6 currently functions as a copy of Tx V5 (including tests); its behavior will be updated in future PRs to reflect the true V6 specification, incorporating ZSA-specific features and changes.
• Placeholder/dummy values are used for activation height, group ID, etc., which will be replaced with proper values in future updates.

Notes

• This implementation provides a foundational structure for Tx V6 but does not yet include ZSA-specific features, new transaction structures, or detailed serialization/deserialization changes.
• Future work will focus on completing Tx V6 support with ZSA-specific elements, proper configuration values, and full integration into the consensus layer.

Next steps

• Address any FIXME comments in the code.
• Convert zebra-chain::orchard::ShieldedData struct to be a generic that supports both V5 and V6.
• Implement serialization/deserialization for V6 (previous work from the /tx_v6_with_ser_1.8.0 branch can be utilized).
• Update Tx V6 to perform its intended behaviors distinct from V5 in subsequent PRs.
• Implement verification processes for Tx V6 transactions.
• Add tests for Tx V6 transactions.