[Process] Add validation on Process input

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT

This adds validation on Process input. For the moment, passing a stream would result in a PHP error.
I propose to deprecate values that are not strictly string in 2.6 (see upcoming PR)