Linq2db: .net core2.2 mysql [DataConnection.DefaultSettings] throw an Exception

[babywzazy]

Use .net core2.2, reference demo,
use [DataConnection.DefaultSettings = new Settings();]
throw an exception

Exception message:Could not load file or assembly 'System.Data.SqlClient, Version=4.5.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
Stack trace:
{System.TypeInitializationException: The type initializer for 'LinqToDB.Data.DataConnection' threw an exception. ---> 
System.TypeInitializationException: The type initializer for 'LinqToDB.DataProvider.SqlServer.SqlServerTools' threw an exception. ---> 
System.IO.FileLoadException: Could not load file or assembly 'System.Data.SqlClient, Version=4.5.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. 
The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

Settings.cs
```c#
public class ConnectionStringSettings : IConnectionStringSettings
{
public string ConnectionString { get; set; }
public string Name { get; set; }
public string ProviderName { get; set; }
public bool IsGlobal => false;
}

public class Settings : ILinqToDBSettings
{
public IEnumerable DataProviders => Enumerable.Empty();

public string DefaultConfiguration => "MySql";
public string DefaultDataProvider => "MySql";

public IEnumerable<IConnectionStringSettings> ConnectionStrings
{
    get
    {
        yield return new ConnectionStringSettings
        {
            Name = "MySql",
            ProviderName = "MySql",
            ConnectionString = "Server=xxx;Port=3306;Database=xxx;Uid=root;Pwd=xxx;charset=utf8;"
        };
    }
}

}

Startup.cs
```c#
DataConnection.DefaultSettings = new Settings();

Environment details

linq2db version: 2.7.0
Database Server: E.g. Windows Server 2008R2
Database Provider: E.g. MySql
Operating system: E.g. Windows 10
Framework version: .NET Core 2.2

[MaceWindu]

Right now System.Data.SqlClient is a direct dependency of linq2db. Probably it's time to get rid of it as it also creates issue for xamarin

[igor-tkachev]

[sdanyliv]

Install System.Data.SqlClient version 4.5.1 or latest to your project and it should work. Never mind that this is for MySql provider.
We will fix this broken dependency.

[MaceWindu]

Looks like we referenced wrong version of System.Data.SqlClient in nuget. linq2db 2.7.x built using System.Data.SqlClient 2.6.0, but nuget references 4.5.1. As workaround - manually update reference to System.Data.SqlClient to 2.6.0

[MaceWindu]

We will fix dependency version by #1744 in next bugfix release. For explicit dependency removal task tracking use #1715