feature #44484 [Translation] [Loco] Send If-Modified-Since header when possible (Kocal)

This PR was squashed before being merged into the 6.1 branch.

Discussion
----------

[Translation] [Loco] Send `If-Modified-Since` header when possible

| Q             | A
| ------------- | ---
| Branch?       | 6.1 <!-- see below -->
| Bug fix?      | no
| New feature?  | yes <!-- please update src/**/CHANGELOG.md files -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tickets       | Fix #44414 <!-- prefix each issue number with "Fix #", no need to create an issue if none exist, explain below instead -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

This PR is a proposal for #44414.

When pulling translations from Loco, we save the `Last-Modified` response header in each `MessageCatalogue` thanks to the new `CatalogueMetadataAwareInterface`.

When pulling translations one more time from Loco, we send the previous `Last-Modified` response header as `If-Modified-Since` request header if it's available.
If Loco returns a 304, we don't update current translations. If it's not the case, we keep the original behaviour.

_Catalogue metadata_ can be used to store metadata about the catalogue itself and not about translations.
Actually, only `.xliff` format is supported (read/write).

WDYT?

Thanks!

cc @welcoMattic

Commits
-------

052aea5ecc [Translation] [Loco] Send `If-Modified-Since` header when possible

Author: fabpot

Resources/config/translation_providers.php

@@ -51,6 +51,7 @@
                 service('logger'),
                 param('kernel.default_locale'),
                 service('translation.loader.xliff'),
+                service('translator'),
             ])
             ->tag('translation.provider_factory')