fix: encoder no longer treats all pointer fields as optional<T>

[ety001]

Problem

The reflection-based encoder in encodeStruct() was adding a 0x01 "pointer present" flag before every pointer field, assuming all Go pointers map to Steem C++ optional<T>. This is incorrect — many operations use plain (non-optional) structs.

For example, witness_update_operation::props is a plain chain_properties in C++, but the Go type uses *ChainProperties. The extra 0x01 byte corrupted the transaction digest, causing steemd to reject the signed transaction with missing required active authority.

Root Cause

In encoder/encoder.go encodeStruct():

// OLD: all pointer fields get a presence byte
if field.Kind() == reflect.Ptr {
    if field.IsNil() { encoder.EncodeNumber(uint8(0)); continue }
    encoder.EncodeNumber(uint8(1))  // ← erroneous flag
    fieldValue = field.Elem().Interface()
}

Fix

• Add a steem:"optional" struct tag to distinguish truly optional fields
• Only emit the presence byte for fields tagged steem:"optional"
• Untagged pointers serialize directly (follow pointer, no flag)
• Tagged AccountUpdateOperation and AccountUpdate2Operation authority fields as optional (matching their C++ optional<authority> definition)
• All other pointer fields (witness_update.props, account_create.owner/active/posting, reset_account.new_owner_authority, escrow_transfer.ratification_deadline/escrow_expiration, limit_order_create.expiration, create_proposal.start_date/end_date, pow.*) are left untagged — they are plain structs in C++

Verification

$ go test ./...
ok  github.com/steemit/steemutil	0.011s
ok  github.com/steemit/steemutil/protocol	0.010s
ok  github.com/steemit/steemutil/transaction	0.048s
...

Added TestWitnessUpdateSerialization — verifies the serialized output is 131 bytes (not 132) and that the byte after block_signing_key is NOT 0x01.