Add support for consuming shell_source through pants run/adhoc_tool

[chrisjrn]

Currently, shell_command is the only way to run shell code as a build step. While it's possible to add a shell_source as a dependency and make the command string simply run the dependency, in practice it encourages writing multi-line shell scripts in BUILD files, where they can't be checked with linters/formatters/testing tools.

Instead, we should support running shell_source targets through adhoc_tool.

Note that this will require adding some sort of support for the tools field, probably on shell_source itself. Work done in #18168 should make this easy enough to implement, but it would also fall out of a good solution to #18320.