Support DataAnnotations Validation

[gao-artur]

There was a request in the past that was closed due to inactivity: #51.
It would be great to add support for the validation attributes defined in the System.ComponentModel.DataAnnotations namespace.

For example:

[ProtoContract]
public class Person
{
    [Required]
    [MinLength(2)]
    [ProtoMember(1)]
    public string FirstName { get; set; }

    [Required]
    [MinLength(2)]
    [ProtoMember(2)]
    public string LastName { get; set; }
}

Sending null value in the FirstName and a in LastName properties should produce an error similar to what Asp.Net Core does when sending a REST request:

{
  "errors": {
    "LastName": [
      "The field LastName must be a string or array type with a minimum length of '2'."
    ],
    "FirstName": [
      "The FirstName field is required."
    ]
  },
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
  "title": "One or more validation errors occurred.",
  "status": 400,
  "traceId": "00-8b40c8ee54cacbefc8bf5338c0c7c809-51bfdad1f249172a-01"
}

Ideally, it should be possible to enable the validation on the server and client sides (why send the request that for sure will fail the validation?)

Alternative solution:
Use 3rd party libraries to enable validations with interceptors, like grpc-aspnetcore-validator