Istio-must-gather doesn't use correct file extension for some files

Some custom files captured from istiod or proxy don't have any file extension, which makes it harder to view these files in IDEs (no syntax highlighting, etc.).

These files are:

• .../controlplane/synchronization
• .../envoyConfiguration
• .../pilotConfiguration

Additionally, for someone familiar with the pilot/proxy debug endpoints, it's not immediately clear what these files are. It would be better to name these files according to the endpoint used to create them (e.g. pilot_config_dump, proxy_config_dump, proxy_stats)