Allow configuring npmClient on a project level

Currently, the npmClient ionic uses can only be set globally, using e.g.:
```sh
ionic config set -g npmClient pnpm
```

In my opinion, it should be possible to override this on a project level, e.g.:
```sh
ionic config set npmClient pnpm
```
This commands "succeeds" and updates the `ionic.config.json`, but ionic doesn't check that file for this parameter (a bit confusing).

The use cases:
- We have some older projects that use npm, and newer once use pnpm. It has happened multiple times that I run `ionic build` when i just switched project and forgot to switch package manager. This then creates a `pnpm-lock` file in an `npm` project. And regularly breaks my dependencies and requires deleting `node-modules` and reinstalling. This wastes time.
- We have pipelines that build iOS projects on local MacMini's. However, if the global ionic package manager changes, and a pipeline doesn't have the setting to change it back, the build breaks. It's not always obvious that the issue is caused by a switch in package manager.

This was previously suggested in https://github.com/ionic-team/ionic-cli/issues/4570, but was not implemented because the lack of use cases.

Start by tracing the existing `ionic config set` handling for global settings and the project-level `ionic.config.json`, then follow how `ionic build` selects its package manager. Confirm the desired precedence between project and global configuration, and verify that builds use the project setting without creating the wrong lockfile.

Escrito por el modelo de indexación a partir del texto del issue.