Provide a UI for configuring the path to the Composer executable

Problem/Motivation

In manually testing this module, I discovered that my web server, nginx, doesn't have Composer available in its PATH. That's all well and good, because we have a documented procedure for configuring the path to Composer.

But you have to edit settings.php to do this, similar to other settings and configuration that is documented in sites/default/default.settings.php. Once this module is in core we could presumably add this as an example you could uncomment in default.settings.php.

You could also install Drush and do something like drush cset package_manager.settings executables.composer /path/to/composer, or you set it manually in config and import, or...probably other ways.

We should determine if editing settings.php will be too difficult for AutoUpdates target audience. The error message is probably scary enough to people who aren't very technical; the path to remediate the error is even scarier. This is not good. We should provide a UI for configuring the path to Composer -- a simple configuration form.

I understand that there are security implications there -- you're literally configuring the path to an executable which we will run -- so I'm not yet calling this issue a stable blocker until we have confirmation from the security team that it's okay to provide a UI like this, and what the appropriate hardening might be (maybe we should provide the UI but only make it accessible to user 1).

Steps to reproduce

In an environment where the PHP interpreter doesn't have Composer in its PATH, install this module. You'll get the error right away and have no way to fix it except with the methods I mentioned above, or some other techy thing.

Proposed resolution

Add a UI to configure the path to Composer.

Remaining tasks

Get the blessing of the security team and decide on what, if any, hardening there should be around this UI. Then build a configuration form for this with a simple functional test.