Remove output type specification by clients

[ola-rozenfeld]

Currently, the client needs to specify on each output whether it is an output_file or an output_directory. This turned out to be too restrictive -- some build tools don't necessarily know what an action produces until it is done.

We could fix this as a non-breaking change in V2, it will be a bit challenging, but possible, if needed (add a new outputs field, keep supporting all fields on the servers, stop type-checking the outputs on the servers, change the clients once servers add support for the new field).

But it will be much simpler to just change this in v3.