Migrate all json config files to use snake case property names

[kaidaguerre]

John Smyth:

That is our 'standard' for the steampipe cloud api.  It seems like we have a mix in steampipe today - versions.json and update-check.json are lowerCamel, plugin_manager.json and steampipe.json are UpperCamel...  Not urgent, but we should probably migrate to a standard format.

We should also look at updating the names for consistency of - vs _ (see also #402)
Files to update:

versions.json
update-check.json
plugin_manager.json
steampipe.json
.mod.cache.json

[kaidaguerre]

type Migrateable interface{
      MigrateFrom(old interface{})
}


func Migrate(old interface{}, new Migrateable, filepath)Migrateable{
     // deserialise into new
     if structVersion == expected{ return}
 
     // deserialise into old
     new.MigrateFrom(old)

   // serialise to file
     return new
}