add namespace parameter to PhpDumper::dump
merged branch zakharovvi/dependencyinjection_phpdumper_namespace (PR #8119)

This PR was submitted for the 2.3 branch but it was merged into the master branch instead (closes #8119).

Discussion
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[DependencyInjection] add namespace parameter to PhpDumper::dump

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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97320a5 [DependencyInjection] add namespace parameter to PhpDumper::dump