Allow merging Args objects

Currently if you want to support persistent workers, you must pass a single Args object to your run action. This args file is read and passed as the WorkRequest to the persistent worker. Any other arguments are passed to the persistent worker as launch arguments.

If you build up your arguments in multiple steps into multiple [Args](https://docs.bazel.build/versions/master/skylark/lib/Args.html) objects, there is no way to combine them to end up passing them all in a single object.

### Feature requests: what underlying problem are you trying to solve with this feature?

Currently rules_swift builds up 2 Args objects, one [here](https://github.com/bazelbuild/rules_swift/blob/c8138ccf8cd798aefc9533840fcab72546e4c46c/swift/internal/api.bzl#L676) and another [here](https://github.com/bazelbuild/rules_swift/blob/c8138ccf8cd798aefc9533840fcab72546e4c46c/swift/internal/api.bzl#L471-L479) it then [passes both objects](https://github.com/bazelbuild/rules_swift/blob/c8138ccf8cd798aefc9533840fcab72546e4c46c/swift/internal/api.bzl#L507) to run. It's nice to build these up separately using the Args API, but supporting persistent workers would require combining these, instead of passing them both.

### What operating system are you running Bazel on?

macOS

### What's the output of `bazel info release`?

0.17.1

Start by reading the Args API references and the rules_swift call sites in swift/internal/api.bzl, especially the sections around lines 471-479, 507, and 676. Determine how multiple Args objects could be combined for a run action and what persistent-worker behavior must be preserved; done means a single combined object can be passed without losing arguments.

Written by the indexing model from the issue text.