Fix ripple explorer (de)serialization issues

[jcoatelen-ledger]

Fixes VSD-2389 and VSD-2405

Changes:

• ledger_index_min is now returned as a integer object and not a string-integer ; the synchronization fails at fetching the account transactions. Fix: change ledger_index_min type from string to unsigned int64
• Amount attribute is missing for AccountDelete tx type. Fix: parse DeliveredAmount, which is there for this tx type instead.

Note: I would not suggest to use delivered_amount which seems always returned because I don't know how the parsing would go for other tx types, e.g. if delivered_amount is always matching Amount (even if I suppose they will be always equal).

Results: sync successful, no error at sync, correct amount saved in db

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"