[Translation] [Xliff] Support for <note>

| Q | A |
|---|---|
| Bug fix? | no |
| New feature? | yes |
| BC breaks? | no |
| Deprecations? | no |
| Tests pass? | yes |
| Fixed tickets | #10550 |
| License | MIT |
| Doc PR | n/a |

The purpose of this PR, is to avoid that the automated translation update (`app/console translation:update`) remove the `note` tags from XLIFF files.


Not sure if using the metadata bag is a good thing or if I should have add a "note" property in MessageCatalogue. What do you think ?