Provide a way to redirect stdout/stderr into a file

### Description of the problem / feature request:

It is common for programs in unix world to output the result of an invocation to stdout. The current way to capture that output is to either use `ctx.actions.run_shell()` or a wrapping shell script to redirect it to a file. This requires spawning an extra process and hence adds some overhead. Perhaps it makes sense to provide a way for `ctx.actions.run()` to redirect stdout and/or stderr into a file.

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

I have a tool that does what I described above - writes the result of its invocation to stdout. I need to capture that output and put it into a file - it is the output of the rule. Currently I have to wrap it in a shell script and I thought it might be good to suggest this feature. Thanks.

Start by reviewing the existing ctx.actions.run() and ctx.actions.run_shell() entry points and the issue discussion about capturing stdout and stderr. Define the intended redirection behavior and API before implementing it, then verify that the requested output can be captured without a wrapping shell process.

Written by the indexing model from the issue text.