feat: user api keys for MEFE interaction; get units API

[nbiton]

Functional changes

1. The internal "CREATE_USER" enterprise API now returns a user API key for the created user.
2. When "CREATE_USER" is called for an existing user, a new user API key is generated and returned in the response. Previous API keys are not revoked as a result, and the "creatorId" provided with the request is recorded as the issuer/generator of the API key.
3. GET /api/units has been made available. It expects apiKey as a query param. It checks if the apiKey is valid, associated with a user and not revoked, before returning a JSON array of object of the following form:

   {
       "_id": String (ObjectId),
       "displayName": String,
       "unitType": String (enum),
       "createdAt": String (Date in ISO format),
       "streetAddress": String,
       "city": String,
       "zipCode": String,
       "state": String,
       "country": String,
       "moreInfo": String
   }

This PR should resolve #773

[nbiton]

The example request relevant to the new endpoint is named "Get accessible units' meta data for a given user with api key" on our Postman account

[nbiton]

The new attribute returned from the "CREATE_USER" internal API is named mefeApiKey. It is a 24-character long string.