As a PMM User I need to specify socket for mongodb to be used in pmm-admin add command so that PMM uses a socket instead of port

Use case:

pmm-admin add mongodb .... --socket=....

DOC: flag present in the documentation 

DOD: 

Ability to connect to MongoDB servers via sockets with pmm-server API.
mongodb_exporter and QAN MongoDB profiler agents should be able to connect to MongoDB using socket.
new flag --socket in `pmm-admin add mongodb` command

Suggested implementation:

1. Add string parameter `socket` to pmm-managed `inventory/services/AddMongoDB` and `management/MongoDB/Add` endpoints requests.
2. Add extra field to services table.
3. Make address and port fields optional and check if only one of two fields(socket, address) exists in API and DB.
4. if `socket` is not empty DSN should use "unix" as network