Add flag --direct-connection to pmm-admin

As per https://docs.percona.com/percona-monitoring-and-management/setting-up/client/mongodb.html#on-the-command-line:

Atlas doesn’t support direct connections. When connecting to an Atlas instance, use the pmm-admin option --direct-connection=false. (Doing so will prevent replicaset status from working and the MongoDB Overview dashboard widget will show invalid values.)

However the option --direct-connection=false is not implemented in pmm-admin:

# pmm-admin add mongodb --username=mongodb_exporter --password=<redacted> --host=127.0.0.1 --port=27017 --direct-connection=false
pmm-admin: error: unknown flag --direct-connection