rollback of metastore is far from perfect

rolling back the metastore is currently not guaranteed to work well:

• assuming the postgresql metastore, we have migrations, which handles column creation/deletion. However if new fields are added in one of the stored json, an old quickwit might not be able to read the content of the metastore (we forbid unknown fields in many places).
• assuming a file based metastore, we don't even have a notion of rollback, except for backing up the metastore files, and restoring them

we should figure out how to make this better. The solution would likely be something along the line of:

• for postgresql: add code in the down migrations to remove the unwanted fields stored inside json (and document the rollback process)
• for other metastore, provide some tool to convert newer metastore files into older ones, and document how to use them to rollback an existing cluster

Start by mapping the PostgreSQL down migrations and the file-based metastore behavior described in the issue. Define the rollback process for both metastore types, including JSON-field compatibility, conversion or restoration steps, and documentation that explains how to roll back an existing cluster.