Fix retrieving namespace from user-defined YAML config

[tatiana]

If users set the value of ray_cluster_yaml pointing to a RayCluster that would have a namespace, such as:

apiVersion: ray.io/v1
kind: RayCluster
metadata:
  name: airflow-raycluster
  namespace: non-default

And

  workerGroupSpecs:
    template:
      metadata:
        namespace: non-default

The RayCluster was still being created in the "default" namespace, since this value was hard-coded in the provider RayHook.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.55%. Comparing base (c52202a) to head (f0548b3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
+ Coverage   98.52%   98.55%   +0.02%     
==========================================
  Files           7        7              
  Lines         610      622      +12     
==========================================
+ Hits          601      613      +12     
  Misses          9        9              

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