Mixins templatize clusterlabel fixes

[hervenicol]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #6352

Special notes for your reviewer:

After the mess from PR #6353, here is a revised version.

The changes from the previous PR:

• support for a config.libsonnet file for promtail-mixins
• added a comment to the new parameter per_cluster_label in loki-mixins config file
• added a gitignore for promtail-mixins vendor/ directory

To test the build failure / fix, I had to locally update my production/promtail-mixin/jsonnetfile.json file:

   {
     "source": {
       "git": {
         "remote": [-"https://github.com/grafana/loki.git",-]{+"https://github.com/hervenicol/loki.git",+}
         "subdir": "production/loki-mixin"
       }
     },
     "version": [-"main"-]{+"mixins_templatize_clusterlabel_fixes"+}
   }

The following build targets were run on my machine, from their respective CI docker images:

• loki-mixin
• loki-mixin-check
• fmt-jsonnet
• lint-jsonnet (the one which was failing)

I hope now everything is OK 🤞

Checklist

• Documentation added
• Tests updated
• Is this an important fix or new feature? Add an entry in the CHANGELOG.md.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[grafanabot]

./tools/diff_coverage.sh ../loki-main/test_results.txt test_results.txt ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki

Change in test coverage per package. Green indicates 0 or positive change, red indicates that test coverage for a package fell.

+           ingester	0%
+        distributor	0%
+            querier	0%
+ querier/queryrange	0%
+               iter	0%
+            storage	0%
+           chunkenc	0%
+              logql	0%
-               loki	-0.7%

[dannykopping]

LGTM @hervenicol - thanks for your contribution!