Свой источник конфигурации Entity Framework

ConfigValue

public class ConfigValue
{
    public string Key { get; set; }
    public string Value { get; set; }
}

EfConfigurationProvider

public class EfConfigurationProvider : ConfigurationProvider
{
    public Action<DbContextOptionsBuilder> OptionsAction { get; }

    public EfConfigurationProvider(Action<DbContextOptionsBuilder> optionsAction)
    {
        OptionsAction = optionsAction;
    }

    public override void Load()
    {
        var builder = new DbContextOptionsBuilder<ExampleContext>();
        
        OptionsAction(builder);
        
        using var dbContext = new ExampleContext(builder.Options);
        Data = dbContext.Configurations.ToDictionary(x => x.Key, v => v.Value);
    }
}

EfConfigurationSource

public class EfConfigurationSource : IConfigurationSource
{
    private readonly Action<DbContextOptionsBuilder> _optionsAction;
    
    public EfConfigurationSource(Action<DbContextOptionsBuilder> optionsAction)
    {
        _optionsAction = optionsAction;
    }
    
    public IConfigurationProvider Build(IConfigurationBuilder builder)
    {
        return new EfConfigurationProvider(_optionsAction);
    }
}

EfExtensions

public static class EfExtensions
{
    public static IConfigurationBuilder AddEfConfiguration(this IConfigurationBuilder configurationBuilder,
                                                           Action<DbContextOptionsBuilder> optionsAction)
    {
        return configurationBuilder.Add(new EfConfigurationSource(optionsAction));
    }
}

ExampleContext

public class ExampleContext : DbContext
{
    public DbSet<ConfigValue> Configurations { get; set; }
    
    public ExampleContext(DbContextOptions options) : base(options)
    {
    }
}

Использование

WebHost.CreateDefaultBuilder(args)
       .ConfigureAppConfiguration((context,
                                   builder) =>
        {
            var config = builder.Build();
            builder.AddEfConfiguration(optionsBuilder =>
            {
                optionsBuilder.UseNpgsql(config.GetConnectionString("DatabaseConnection"));
            });
        })
       .UseStartup<Startup>();