[esArchiver] v6 kibana index support

[spalger]

Requires #12794

When we remove support for v5 kibana indices we will need to update our es archives to use v6 format. To do that we add a convertToV6Stream to the esArchiver that converts doc and index records to v6 format on the fly. This enabled:

• loading v5 data files with the load command, which write the data into elasticsearch in v6 format which powers...
• the new reindex command which will read data from elasticsearch in v5 format and put it back into elasticsearch in v6 format
• and allows the rebuild-all command to read the archives from disk in v5 format and write them back to disk in v6 format

As always, use node scripts/es_archiver --help for CLI syntax. For the impatient, the following will read the .kibana index (or any indices specified after reindex) to a tmp directory on disk, then delete the indices, and recreate them by first converting the docs into v6 format

node scripts/es_archiver reindex .kibana 

The existing archives were also updated to use v6 format using the rebuild-all command.

[spalger]

Merged esArchive updates into #12794, since reindex command would only be useful for a hot minute we aren't going to merge it into the project. If devs want to migrate existing kibana indices to v6 format they can use the upgrade UI in 5.6. Otherwise starting from a fresh kibana index will do the trick