Add support for flag enums with more than 32 values

[genne5]

The generator has so far only generated enums with the implicit default type int.
Also for enums declared with the base type long in C# and a "Format = int64" was specified in the OpenAPI document.
Original C#

 public enum Foo : long
    {
        _0 = 0L,
        _1 = 1L << 1,
        _2 = 1L << 2,
       ...
        _31 = 1L << 31,
        _32 = 1L << 32,
        _33 = 1L << 33
    }

Swagger Definition

      "Foo": {
        "type": "integer",
        "format": "int64",
        "description": "",
        "x-enumNames": [
          "_0",
          "_1",
          "_2",
       ...
          "_31",
          "_32",
          "_33"
        ],
        "enum": [
          0,
          2,
          4,
       ...
          2147483648,
          4294967296,
          8589934592
        ]
      }

NSwag Output

 public enum Foo
    {
        _0 = 0,
        _1 = 2,
        _2 = 4,
       ...
        _31 = 2147483648,
        _32 = 4294967296,
        _33 = 8589934592
    }

The nswag output does not compile because "long" is missing as base type.
The values exceed the range of the implicit base type int.