feature: Add attribute fileName

[wndhydrnt]

Current implementation creates the name of the file to create by concatenating the attributes application, profile and fileType.
Example:

application: "myapp"
profile: "prod"
fileType: "json"

becomes myapp-prod.json.

This is hard for users to grasp. People often are confused and don't know how to reference the file in the filesystem of a container.

This change adds the attribute fileName and deprecates the attribute fileType.
Users can define the name of the file. However, the extension of the file needs to be supported by Spring Cloud Config Server.
The supported extensions are .yaml, .yml, .json and .properties.