Introduce priority for fields.go (elastic#9703)

The problem with fields.go is that the initialisation is based on the init() statement. It is not fully predicable the order the init statements are executed. But for the fields it is important that they are in the right order if fields should be overwritten. To ensure this two things were introduce: Priority to ensure libbeat fields are added first, alphabetical sorting to make sure the generated content is always the same.

More work is need as soon as the ECS and processor files have their own fields.go files to also assign them different priorites.