Please add Retry-After header when serving app_offline.htm

[naveedahmed1]

Further to discussion at https://github.com/aspnet/IISIntegration/issues/204#issuecomment-334291258

Please add Retry-After when serving app_offline.htm since search engines suggest adding it while performing site maintenance.

Ref: https://webmasters.googleblog.com/2011/01/how-to-deal-with-planned-site-downtime.html

May be header with some default value like 15 minutes with some option to customize it further through web.config (attribute of the aspNetCore element).