Replace Hangfire with custom CRON job runner

[ostorc]

Closes #208

Todo:

• migrate Hangire jobs
• add some statistics for jobs (not sure yet)

[ostorc]

Example implementations:

Scoped job:

[Cron("*/10 * * * * *")] // Not mandatory, you can register it with cron explicitly
public class WarningCountJob : IJob
{
    private readonly IWarningService _warningService;
    private readonly ILogger<WarningCountJob> _logger;
    public string Name { get; } = "Warning Count Job";

    public WarningCountJob(IWarningService warningService, ILogger<WarningCountJob> logger)
    {
        _warningService = warningService;
        _logger = logger;
    }

    public async Task ExecuteAsync(CancellationToken cancellationToken = default)
    {
        var warnings = await _warningService.GetAllWarningsAsync();

        _logger.LogError("Total warning count: {WarningCount}", warnings.Count);
    }
}

Singleton job:

public class PingJob : ICronJob
{
    private readonly ILogger<PingJob> _logger;
    public string Name { get; } = "Ping";
    public string CronExpression { get; } = "*/20 * * * * *";

    public PingJob(ILogger<PingJob> logger)
    {
        _logger = logger;
    }

    public Task ExecuteAsync(CancellationToken cancellationToken = default)
    {
        _logger.LogWarning("Ping at {Now}", DateTime.Now);
        return Task.CompletedTask;
    }
}

[ostorc]

How to configure services:

services.AddScheduler(5000)
    .AddCronJob<PingJob>()
    .AddScopedCronJob<WarningCountJob>("*/20 * * * * *") // we can explicitly use cron expression
    .AddScopedCronJob<WarningCountJob>(); // or use CronAttribute

[tenhobi]

We are putting contracts to the same package now?

// #175

[ostorc]

We are putting contracts to the same package now?

// #175

Mainly, because I didnt want to add another package only with three files... And since this is fairly lightweight library, I dont think it will be issue.

[tenhobi]

LGTM