feat(cache): add shared S3-compatible storage (#1244)

Store the cache used by `actions/cache` in an S3-compatible bucket such as AWS S3 or RustFS, with no `cache-server`. Runners with the same settings use the same cache.

A cache saved on one runner can be restored on the others within 30 seconds. The runner talks to the bucket over the S3 REST API with SigV4, without an SDK. Lifecycle rules on the bucket expire old caches.

*Description rewritten by Claude.*

---------

Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1244
Reviewed-by: silverwind <[email protected]>
Co-authored-by: bircni <[email protected]>

internal/app/cmd/cache-server.go

@@ -29,6 +29,9 @@ func runCacheServer(configFile *string, cacheArgs *cacheServerArgs) func(cmd *co
 		}
 
 		initLogging(cfg)
+		if cfg.Cache.S3 != nil {
+			return errors.New("cache-server does not support cache.s3; set cache.s3 on the runners instead")
+		}
 
 		var (
 			dir  = cfg.Cache.Dir