Support to decode metadata in the API that list ledgers

[fantapsody]

FEATURE REQUEST

1. Please describe the feature you are requesting.

The current list ledgers API output the metadata in a serialized binary format, which is not friendly to human operators and external tools, and is not consistent with the output of the API that gets the metadata. For example:

curl 'localhost:8080/api/v1/ledger/list?print_metadata=true'

{
  "0" : "BookieMetadataFormatVersion\t3\n(\b\u0001\u0010\u0001\u0018��\u0006 �\u0007(\u00032\u0011\n\r10.0.0.9:3181\u0010\u00008\u0003B\u0000H\u0001`\u0000"
}

2. Indicate the importance of this issue to you (blocker, must-have, should-have, nice-to-have).
   Are you currently using any workarounds to address this issue?
   This is a should-have feature, otherwise, all eternal tools rely on this API have to deserialize the metadata by themselves, which is much harder than to parse a plaintext JSON object.

3. Provide any additional detail on your proposed use case for this feature.

Add a parameter such as decode_meta, and output the ledger metadata in decoded format when the parameter presents and the value of it is 'true'.