Refactor shared ledger/request common fields

[e-desouza]

Summary

Track the broader CommonFields refactors raised during PR #154 review, but intentionally kept out of the XLS-70 credentials implementation because they affect existing public model/request shapes across the crate.

Review comments

• Ledger objects: move common ledger object fields such as PreviousTxnID and PreviousTxnLgrSeq into ledger-object common fields.
  • feat: add Credentials support (XLS-70) #154 (comment)
• Requests: consider moving LookupByLedgerRequest into request CommonFields.
  • feat: add Credentials support (XLS-70) #154 (comment)

Scope

• Audit ledger object models that duplicate common metadata fields such as PreviousTxnID and PreviousTxnLgrSeq.
• Decide whether those fields should move into src/models/ledger/objects/CommonFields or another shared ledger-object metadata type.
• Audit request models using LookupByLedgerRequest.
• Decide whether ledger lookup fields should move into request CommonFields or remain as an explicit flattened request component.
• Preserve serde compatibility with rippled JSON field names.
• Document any public API impact and update tests/fixtures accordingly.

Notes

This should be handled separately from PR #154 because it is a cross-model API refactor rather than an XLS-70-specific correctness fix.