fix(DumpHistoryRequest): allow fetching from genesis

[SebastienGllmt]

For utxorpc, startToken is optional and, if omitted, will fetch history starting from the genesis block

However, the typescript SDK accidentally made this a required field

You can test this yourself if you want to verify the behavior when startToken is omitted with the following query

grpcurl -plaintext -d '{
    "maxItems": 3
}' localhost:50051 utxorpc.v1alpha.sync.SyncService.DumpHistory