Enhancement: Support further environment variables for swift authentication

The swift backend now supports the following additional environment variables
to pass authentication details to restic: `OS_USER_ID`, `OS_USER_DOMAIN_ID`,
`OS_PROJECT_DOMAIN_ID` and `OS_TRUST_ID`.

Depending on the openrc configuration file these might be required when the
user and project domains differ.

https://github.com/restic/restic/issues/3147
https://github.com/restic/restic/pull/3158