Fix ledger explorer client

[etschelp]

In the wallet under Unverified Information of Your Organization you can request verification. This partner list is populated with the help of a ledger explorer. The LedgerExplorerClient class uses the format that is returned from the explorer that is based on the von-image. The current issue is that the preconfigured test network (test.bcovrin.vonx.io) is too slow so that it does not return any results before a timeout, and for the idunion network there is only a indyscan explorer publicly available which works different.

This can be fixed in the following ways:

1. Host own test ledger again, host von-image based ledger explorer for the idunion network again
2. Make the LedgerExplorerClient an interface, add a ledger explorer type to the config and inject a conditional bean depending on it. This also requires writing a new implementation for the indyscan type explorer. This does not fix the slow explorer though, which means that either an own test ledger has to be deployed again, or an additional indyscan explorer needs to be setup pointing to the bcovrin test ledger.