Support automatic discovery of license keys

[jbogard]

Today AutoMapper requires manual setting of the license key:

services.AddAutoMapper(cfg => {
    cfg.LicenseKey = "License key here";

    // Other configuration
});

While this allows for unlimited options for retrieving the license key (typically via the .NET configuration system), it still requires manual setting. This can be problematic for larger enterprises that share the license key amongst many systems/applications. This also requires code changes to set the license key.

Instead, we can support automatic discovery of license key, via:

• .NET configuration system (via a well-known key)
• Environment variables
• File system?

This should support registration via IServiceCollection and manual creation.

This may simplify some overloads back to releases prior to 15.x since a license key could be dynamically discovered instead of required in the configuration system.