CIP-129 header byte should allow inferring DRep type

[linconvidal]

Context

• Network: preprod
• Endpoint: /construction/preprocess
• Related discussion: Add Vote Delegation Operation to Rosetta (Installment 1) #288 (comment)

Description

Per the original spec discussion in #288, the drep.type field should be optional when a CIP-129 compliant hex identifier is provided. The CIP-129 format includes a header byte that encodes the credential type:

• 0x22 = DRep Key Hash (bits [7;4]=0010 DRep, bits [3;0]=0010 key hash)
• 0x23 = DRep Script Hash (bits [7;4]=0010 DRep, bits [3;0]=0011 script hash)

Example from test:

"drep": {
  "id": "22f72050aa252ccc6bf75747184910c0b9386298167656f935d6b6c26a"
}

Here 22 indicates a DRep with key hash credential, so the type field should be inferable.

Current behavior

{
  "code": 5000,
  "message": "An error occurred",
  "details": {
    "message": "[operations[2].metadata.drep.type must not be null]"
  }
}

Expected behavior

When drep.id contains a CIP-129 hex identifier with header byte, the API should infer the type automatically and not require the type field.