Add ResponseBuilder.(newShardsInfo|addShardInfo) methods to facilitate 'shards.info' content customisation.

SOLR-13511 previously added a SearchHandler.newResponseBuilder method to facilitate custom plugins' maintenance of per-request state in a custom ResponseBuilder.

The factoring out of two ResponseBuilder methods would support shards.info customisations such as returning of a list instead of a map container and/or removal or redaction of some shards.info elements.