Create new service method to translate find-and-replace operations to batch specs

Created by: courier-new

Pending a final decision on https://github.com/sourcegraph/sourcegraph/issues/39192, we should define a new backend in the service layer which takes as inputs the batch change, batch spec configuration (branch, changeset template, etc.), compute search query, find term, and replace term and creates a functional batch spec out of them.

If we choose to skip using sed and jump straight to a custom Go CLI tool and a different image, this issue does not include implementation of that part, only writing a batch spec as if it were to use those tools.

More details TBD.