Authentication for database backup

__Proposal:__
In order to create a backup of an influxdb2 instance, you need to enable the root token. That is not practical in particular for daily backup as it requires the root token to be always enabled.

__Current behavior:__
Currently, the root token is required [as per documentation](https://docs.influxdata.com/influxdb/v2.0/backup-restore/backup/)

__Desired behavior:__
Find a different way that will not require to have the powermost token to be enabled.

__Alternatives considered:__
Either one of the two:
- Create a dedicated token for backups
- If you have superuser access to the machine, then allow the request to go through with different means of authentication

__Use case:__
In our case we run daily backups with scripts overnight and for that case we must have the root token always enabled, which is something we would like to avoid to improve security.

Start by reviewing the backup documentation linked in the issue and the current requirement for the root token. Compare the proposed dedicated-token and alternate authentication approaches; done means daily backups can run without keeping the root token enabled, with the resulting behavior documented.

Written by the indexing model from the issue text.