Add the API for search files (#15605)

* Adding search files api

* Fixing golangci-lint

* Adding bulk-indexing and improving a bit the name indexing for bleve and elasticsearch

* Add content extraction config migration

* Fixing a problem with document extraction

* Unapplying certain changes moved to other PR

* Fixing tests

* Making extract content app migration a private method

* Addressing PR review comments

* Addressing PR review comments

* Adding feature flag

* Removing debug string

* Fixing imports

* Fixing linting errors

* Do not migrate the config if the feature flag is not enabled

* Fix tests

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

i18n/en.json

@@ -1870,6 +1870,14 @@
     "id": "api.post.save_is_pinned_post.town_square_read_only",
     "translation": "This channel is read-only. Only members with permission can pin or unpin posts here."
   },
+  {
+    "id": "api.post.search_files.invalid_body.app_error",
+    "translation": "Unable to parse the request body."
+  },
+  {
+    "id": "api.post.search_files.not_implemented.app_error",
+    "translation": "This feature is in development, and is only available using a feature flag."
+  },
   {
     "id": "api.post.search_posts.invalid_body.app_error",
     "translation": "Unable to parse the request body."
@@ -8490,6 +8498,10 @@
     "id": "store.sql_command.update.missing.app_error",
     "translation": "Command does not exist."
   },
+  {
+    "id": "store.sql_file_info.search.disabled",
+    "translation": "Searching files has been disabled on this server. Please contact your System Administrator."
+  },
   {
     "id": "store.sql_post.search.disabled",
     "translation": "Searching has been disabled on this server. Please contact your System Administrator."