add `RuleBuilder::then_compute_output` (#4299)

this allows the developer to compute a mocked output using content from
the input.

## Motivation and Context
Some richer use cases for stubbing/mocking require computing a stubbed
output using data from the input, for instance stubbing a method that
returns the item that was put.

## Description
Add a new method to `RuleBuilder` that allows for providing a function
that computes the output. That function takes in a reference to the
input as an argument. I'm not sure how this works with the mutable
inputs like streamed bytes, but seems to work for the simple stuff.

## Testing
added a simple unit test

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Co-authored-by: Aaron Todd <aajtodd@users.noreply.github.com>