Support loading external configuration from files with no file extension

[wilkinsona]

See microsoft/azure-spring-boot#926 and #22278 for background. Azure does not allow . characters in the name of a secret file so you have to jump through a renaming hoop to add the .properties or .yaml suffix.

[philwebb]

We'll look at supporting a new syntax for this. Something like spring.config.location=/etc/application[.properties] where the [.properties] isn't part of the filename, but just a hint for the content type.