fix: encode public_key_type fields as 33-byte binary blobs

[ety001]

Problem

public_key_type fields (like block_signing_key in witness_update,
memo_key in account_create/account_update) were serialized as
varint-prefixed strings by the reflection encoder. In Steem's C++ binary
format, public_key_type is a fixed 33-byte blob (1 byte format + 32 bytes
key data), not a variable-length string.

This caused a digest mismatch on every transaction involving public keys:
the SDK computed the digest using a 54-byte varint-string represe ntation,
while steemd expected a 33-byte binary format.

Fix

• Add steem:"pubkey" struct tag for public_key_type string fields
• Add encodePubKey() to the encoder: parses STM... → wif.PublicKey
  and writes the 33-byte compressed binary
• Tag BlockSigningKey in WitnessUpdateOperation, and all MemoKey
  fields in AccountCreate, AccountUpdate, CreateClaimedAccount,
  AccountUpdate2, AccountCreateWithDelegation operations
• Update golden hex in TestWitnessUpdateGoldenHex (131 → 110 bytes)

Verification

$ make test   # all green

Tested with a live witness signing key update on Steem mainnet —
transaction a840f7c70d3549ac53fcd83ca5740f2092364b03 accepted at
block 107590219.