INSPECT-5605 add float nan validator

[laurence-smith]

Add a float/double validator to optionally prevent NaN values. Default if unspecified is to allow NaN's.

Example Usage:

  double nan_allowed = 1 [(validator.float) = {
    allow_nan: true
  }];
  double nan_disallowed = 2 [(validator.float) = {
    allow_nan: false,
    optional: true
  }];