feat: Add warning message about release version to dnf plugin

* Card ID: CCT-171
* New feature: when a release is set by subscription-manager,
  then dnf plugin "subscription-manager" prints warning about
  this release
* Information about release is read only from cache file.
  When there is cache file containing information about
  current release, then only this information is used
  for printing warning about release lock. Gathering such
  information from candlepin would unnecessarily slow
  down DNF.

src/plugins/dnf/subscription_manager.py

@@ -32,11 +32,12 @@
 
 from configparser import ConfigParser
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Optional
 
 if TYPE_CHECKING:
     from subscription_manager.certdirectory import EntitlementDirectory
     from subscription_manager.identity import Identity
+    from subscription_manager.cache import ReleaseStatusCache
 
 
 expired_warning = _(
@@ -70,6 +71,12 @@
 """
 )
 
+release_lock_warning = _(
+    """
+This system has release set to {release_version} and it receives updates only for this release.
+"""
+)
+
 log = logging.getLogger("rhsm-app." + __name__)
 
 
@@ -206,6 +213,22 @@ def _warn_or_give_usage_message():
                     msg = not_registered_warning
             elif len(ent_dir.list_valid()) == 0 and not is_simple_content_access(identity=identity):
                 msg = no_subs_warning
+            else:
+                # Try to read release version ONLY from cache document.
+                # When cache document does not exist, then do not try to get this information
+                # from candlepin server and slow down DNF plugin!
+                release_cache: ReleaseStatusCache = inj.require(inj.RELEASE_STATUS_CACHE)
+                release_version_dict: Optional[dict] = release_cache.read_cache_only()
+                if release_version_dict:
+                    try:
+                        release_version: str = release_version_dict["releaseVer"]
+                    except KeyError:
+                        log.warning("The 'releaseVer' not included in the release version document")
+                    else:
+                        # Skip the case, when release does not exist at all, or it was unset
+                        # and it is empty string
+                        if release_version:
+                            msg = release_lock_warning.format(release_version=release_version)
         finally:
             if msg:
                 logger.info(msg)