[Dijkstra] replace `TxBody` field `txRequiredGuards : ℙ KeyHash` with `txGuards : ℙ Credential`

[williamdemeo]

Problem

CIP-0112 upgrades required signers to a set of credentials (key or script).
Dijkstra currently uses ℙ KeyHash, which cannot represent required guard scripts.

Proposed change

• Introduce a TxBody field present at both levels:

  • txGuards : … Credential … (container tbd: List, OSet, etc)

• Replace all uses of txRequiredGuards in Dijkstra with the new credential container.

Notes

• Credential already exists (KeyHashObj / ScriptObj) in Ledger.Core.Specification.Address.
• We should also standardize terminology: guards everywhere (not observers).

Acceptance criteria

• Type-checks.
• Any prior use of txRequiredGuards is migrated.
• TxInfo.vkKey still denotes signatories (using KeyHash), while TxInfo.txGuards carries guards (using Credential).