align(sts): use filer /etc/ path convention for IAM storage

- Update DefaultSessionBasePath to /etc/iam/sessions (was /seaweedfs/iam/sessions)
- Update DefaultPolicyBasePath to /etc/iam/policies (was /seaweedfs/iam/policies)
- Update DefaultRoleBasePath to /etc/iam/roles (was /seaweedfs/iam/roles)
- Update iam_config_distributed.json to use /etc/iam paths
- Align with existing filer configuration structure in filer_conf.go
- Follow SeaweedFS convention of storing configs under /etc/
- Add FILER_INTEGRATION.md documenting path conventions
- Maintain consistency with IamConfigDirectory = '/etc/iam'
- Enable standard filer backup/restore procedures for IAM data
- Ensure operational consistency across SeaweedFS components

test/s3/iam/iam_config_distributed.json

@@ -7,7 +7,7 @@
     "sessionStoreType": "filer",
     "sessionStoreConfig": {
       "filerAddress": "localhost:8888",
-      "basePath": "/seaweedfs/iam/sessions"
+      "basePath": "/etc/iam/sessions"
     },
     "providers": [
       {
@@ -42,14 +42,14 @@
     "storeType": "filer",
     "storeConfig": {
       "filerAddress": "localhost:8888",
-      "basePath": "/seaweedfs/iam/policies"
+      "basePath": "/etc/iam/policies"
     }
   },
   "roleStore": {
     "storeType": "filer",
     "storeConfig": {
       "filerAddress": "localhost:8888",
-      "basePath": "/seaweedfs/iam/roles"
+      "basePath": "/etc/iam/roles"
     }
   },