Add support for fields validation

Hi. I'm moving my model from class with properties to read-only struct, and I also want to save the same validations as I have previously.

Unfortunately, the Validator doesn't iterate over fields of the struct and skips all validations.

It would be a call to have validation work for fields as it works for properties.

Example model code where Validator doesn't validate fields

using System.ComponentModel.DataAnnotations;
using System.Text.Json.Serialization;

namespace SpawnMetricsStorage.Models.MetricRecordFiles;

[method: JsonConstructor]
public readonly struct MetricRecord(string name, DateTime logTimeUtc, string commitGitHubUrl, string commitMessage, string value, string units)
{
    [Required]
    // TODO: Combine into one attribute since these are used in a few places
    [MinLength(MetricRecordConstants.MinMetricNameLength)]
    [MaxLength(MetricRecordConstants.MaxMetricNameLength)]
    [JsonInclude]
    public readonly string Name = name;

    [Required]
    [JsonInclude]
    public readonly DateTime LogTimeUtc = logTimeUtc;

    [Required]
    [MinLength(MetricRecordConstants.MinCommitGitHubUrlLength, ErrorMessage = MetricRecordConstants.CommitGitHubUrlShorterErrorMessage)]
    [MaxLength(MetricRecordConstants.MaxCommitGitHubUrlLength)]
    [Url]
    [RegularExpression(@"https:\/\/github\.com\/[^\/]+\/[^\/]+\/commit\/[\da-fA-F]{8}", ErrorMessage = "Invalid GitHub commit URL")]
    [JsonInclude]
    public readonly string CommitGitHubUrl = commitGitHubUrl;

    [Required]
    [MinLength(MetricRecordConstants.MinStringLength)]
    [MaxLength(MetricRecordConstants.MaxCommitMessageLength)]
    [JsonInclude]
    public readonly string CommitMessage = commitMessage;

    [Required]
    [MinLength(MetricRecordConstants.MinStringLength)]
    [JsonInclude]
    public readonly string Value = value;

    [Required]
    [MinLength(MetricRecordConstants.MinStringLength)]
    [MaxLength(MetricRecordConstants.MaxUnitsLength)]
    [JsonInclude]
    public readonly string Units = units;
}