Enhancement: Support additional environment variables for Swift authentication

The `swift` backend now supports the following additional environment variables
for passing 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 from one another.

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