Add Spark join strategy hints to dimension links

[shangyian]

Summary

This enables users to declare a Spark join strategy hint on any dimension link via a new spark_hints field (broadcast, merge, shuffle_hash, shuffle_replicate_nl). When set, DJ emits the hint in a /*+ ... */ comment in the generated SQL, giving the dimension link definer more control over Spark's choice of join algorithm. Multiple hints across different dimension links are combined into a single /*+ BROADCAST(t2), MERGE(t3) */ comment.

Before: no way to influence Spark's join strategy from the semantic layer:

  node.link_complex_dimension("dim.customer", join_on="...", join_type="left")

After: declare broadcast hint on the dimension link

  node.link_complex_dimension("dim.customer", join_on="...", join_type="left",
  spark_hints="broadcast")
  #  SELECT /*+ BROADCAST(t2) */ t2.country, COUNT(t1.order_id) ...
  #  FROM orders t1 LEFT JOIN customer t2 ON ...

The field is available via the REST API, YAML deployment specs, and the Python client. It also round-trips through all node revision copy paths so it is never silently dropped on node updates or redeployments.

Test Plan

Added various tests

• PR has an associated issue: Add Join Hints Support for Dimension Links #1561
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

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