Add support for LEDGER_ENTRY_RESTORE

[tamirms]

Protocol 22 will introduce a new ledger entry change type called LEDGER_ENTRY_RESTORE

When an entry is restored, a new type of LedgerEntryChange is emitted: LedgerEntryChangeType = LEDGER_ENTRY_RESTORE. The entire ingestion stack will need to be able to understand the semantics of this new change.

The ingestion library will need to support LEDGER_ENTRY_RESTORE because it currently returns an error when encountering an unknown change type. Horizon ingestion should interpret a LEDGER_ENTRY_RESTORE change in the same way as LEDGER_ENTRY_CREATED because the two are functionally equivalent.