Support environment variable expansion in config file

[annanay25]

Allow adding sensitive information to the config file by expanding environment variables. Some backend drivers have native support for this (ex: GCS/S3) but there might be other instances where this is desired, for instance in Memcached/Redis/Azure/etc.

The solution is to add a new cli option -config.expand-env that will replace any environment variables denoted by ${VAR} in the config file.

This is inspired by Loki inspired by Cortex :)
grafana/loki#2311
https://cortexmetrics.io/docs/configuration/configuration-file/#use-environment-variables-in-the-configuration

Related: #749

[NickLarsenNZ]

This doesn't work for tempo-vulture