redesign config checker to support freeform keys

[dmachard]

fix #676

Individual function IsValid has been added to the following structs:

• Config
• ConfigGlobal
• ConfigCollectors
• ConfigLoggers
• ConfigMultiplexer
• ConfigPipelines
• ConfigTransformers

The config checker support freeform keys and the code is more easy to maintain!

[dmachard]

@pieterlexis-tomtom I would like to add you as reviewer but I cannot. Any feedback will be appreciated (if you can)

[pieterlexis-tomtom]

I'll have a look, but I must say I am not well-versed with reflect

[dmachard]

No worries, or just some tests will be great too.

[pieterlexis-tomtom]

This seems to work well on a few configs I tried 👍. Good stuff!

[pieterlexis-tomtom]

If I can point to an improvement: You're reading the config file multiple times while loading (first for config, then checking), perhaps you should read it once and use the byte[] to create both the map[string]interface{} and Config struct? That ensures you're always working with the same data, should the file be changed underneath your feet.

[dmachard]

thanks, changed done!