Add entity.FunctionScore carrying both functions and score-option params
(boost_mode / function_mode), and expose it on SearchOption and
HybridSearchOption via WithFunctionScore. The client previously
serialized only FunctionScore.functions, so the combination modes of the
boost scorer were not expressible.
Keep WithFunctionReranker/WithFunctionRerankers append semantics by
lazily building the score, leaving existing callers unaffected.
issue: #45235
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Signed-off-by: Congqi Xia <[email protected]>