Feature: allow custom mnemonic for copy_file

Custom mnemonics can have some useful benefits. For examples, it can allow for more fine-grained filtering of what actions get to run locally vs remotely (via `--modify_execution_info` option / `no-remote` tags).

To that effect, it would be great to allow users to specify custom mnemonics for `copy_file`, via an attribute.

If maintainers are happy with the idea, I'll happily submit a PR.

Start at the copy_file rule implementation and its existing tests, then trace how its action mnemonic is set. Define the custom mnemonic attribute and verify it works with --modify_execution_info and no-remote filtering while preserving the default behavior.

Written by the indexing model from the issue text.