read config file from stdin

currently, the only way to set the full config is by reading files from the filesystem.
however, it's common to read from stdin or from a string.

Example:

# from a file + docker compose exec/run
cat "$CONFIG_PATH" | docker compose exec -T $SERVICE_NAME pbm config --file="-"
# from an env var + docker exec
echo "$CONFIG_CONTENT" | docker exec -i $CONTAINED_ID pbm config --file="-"

The cat "prints" the content of the "$CONFIG_PATH" file to stdout and PBM CLI reads the content (through unix pipeline) from stdin.

The file is not required to be accessible for PBM CLI. PBM CLI can be run remotely and the pipeline allows to send the file via the network.

For example, ssh session: you do not need to copy with `scp` a config file. Just pipeline from local host to remote through opened ssh session.