Ledger/transaction_logic: extract transaction_union_payload module

to separate file

Split the large transaction_logic.rs file by extracting the
transaction_union_payload module (679 lines) into its own file at
ledger/src/scan_state/transaction_logic/transaction_union_payload.rs.

Changes:
- Created transaction_union_payload.rs with TransactionUnionPayload,
  TransactionUnion, Common, Body, Tag types
- Includes timing validation functions (validate_timing,
  validate_nonces, account_check_timing,
  timing_error_to_user_command_status)
- Includes receipt chain hash functions (cons_signed_command_payload,
  checked_cons_signed_command_payload, cons_zkapp_command_commitment)
- Includes helper functions (add_amount, sub_amount,
  get_with_location, set_account, get_account)
- Includes ExistingOrNew and TimingValidation types
- Made helper functions public for use in transaction_partially_applied
- Added module declaration and re-exports in mod.rs for all public
  items
- Added explicit imports including OneOrTwo from
  scan_state::scan_state::transaction_snark
- Removed dangling cfg attribute at end of file
- Added cfg attribute to for_tests module declaration
- Removed duplicate TransactionUnionPayload import from self block

Author: dannywillems