rest: add /server GET and POST methods

DB stores different ogserver addresses in the "entornos" table.

Expose addition and deletion operations for the "entornos" table using
the /server endpoint.

GET /server returns a list of rows from the "entornos" table including
the "identorno" and "ipserveradm" columns. For example:

>>>
GET /server

<<<
200 OK
...
{
  "servers": [
    {
      "id": 1,
      "address": "10.141.10.1"
    }
  ]
}

POST /server inserts into the "entornos" table and returns the
id ("identorno") of the new row.

>>>
POST /server
{
  "address": "192.168.2.240"
}

<<<
200 OK
...
{
  "id": 2
}

If the server address already exists the response is 400 Bad Request
with no payload.

>>>
POST /server
{
  "address": "192.168.2.240"
}

<<<
400 Bad Request

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