Automatically resolve File to base64 encoded value in configuration.

[KrishnaST]

I want to serve binary files as values through configuration from config server.
e.g. The following configuration

keystoreFile='{file}'keystore.jks

Should be resolved to by reading content of mentioned file and converting to base64 before serving to client

keystoreFile=YWJjZGV...

This feature request is based on similar underlying need issues-490