Provide hooks for data refresh and data changed

[colinangusmackay]

Provide hooks for Data Refresh and Data Changed.

e.g.

builder.AddSqlServer(opts =>
{
    opts.FromExistingConfiguration()
        .ExpectLogger()
        .AddOnRefreshedHook( (config, logger) =>
        {
            // Do stuff when refreshed (not necessarily changed).
            logger.LogInformation("The configuration data has been refreshed.");
        }
        .AddOnChangedHook( (config, logger) =>
        {
            // Do stuff when changed.
            logger.LogInformation("The configuration data has changed.");
        }
});