This commit adds a keystore.seed setting that is automatically
generated when the ES keystore is created. This setting may be used by
plugins as a secure, random value. This commit also auto creates the
keystore upon startup to ensure the new setting is always available.

This commit adds a keystore.seed setting that is automatically
generated when the ES keystore is created. This setting may be used by
plugins as a secure, random value. This commit also auto creates the
keystore upon startup to ensure the new setting is always available.

* master: (458 commits)
  Prevent cluster internal `ClusterState.Custom` impls to leak to a client (elastic#26232)
  Add packaging test for systemd runtime directive
  [TEST] Reenable RareClusterStateIt#testDeleteCreateInOneBulk
  Serialize and expose timeout of acknowledged requests in REST layer (elastic#26189)
  (refactor) some opportunities to use diamond operator (elastic#25585)
  [DOCS] Clarified readme for testing a single page
  Settings: Add keystore.seed auto generated secure setting (elastic#26149)
  Update version information (elastic#25226)
  "result" : created -> "result" : "created" (elastic#25446)
  Set RuntimeDirectory (elastic#23526)
  Drop upgrade from full cluster restart tests (elastic#26224)
  Further improve docs for requests_per_second
  Docs disambiguate reindex's requests_per_second (elastic#26185)
  [DOCS] Cleanup link for ec2 discovery (elastic#26222)
  Fix document field equals and hash code test
  Use holder pattern for lazy deprecation loggers
  Settings: Add keystore creation to add commands (elastic#26126)
  Docs: Cleanup docs for ec2 discovery (elastic#26065)
  Fix NPE when `values` is omitted on percentile_ranks agg (elastic#26046)
  Several internal improvements to internal test cluster infra (elastic#26214)
  ...