Implement file loader

[Dakostu]

This PR implements a file loader with an optional --timeout parameter that forwards regular and UDS file input to the next pipeline operator.

This loader also replaces the stdin loader.

[jachris]

You might want to consider overriding operator_base::infer_type_impl for load_operator as part of this PR. This is because the default implementation of infer_type_impl will instantiate the pipeline with a dummy generator, which calls vast::plugins::file::plugin::make_loader in this case. This will already try to open the file which can fail. However, perhaps this part of the pipeline is to be executed on another node where it would succeed.

For the loader, this effect is rather subtle, but for the saver you most definitely want to override it.

Edit: There is another alternative, which is to put the side-effects into the returned generator, such that they will not be executed as part of type checking.

[jachris]

Nice, we are now very close to having a file loader 🎉

Sorry for writing so many comments – most of them are nits or questions. I would like to take one final look after things are clarified.

[jachris]

Great. I think this will be complete after the open points have been addressed 🎉