Optimize block loader of `semantic_text` field for synthetic source

[lkts]

semantic_text field always uses fallback synthetic source (implementation of synthetic source that uses a special _ignored_source field to store raw field data). If such fields are expected to be used with synthetic source it would be beneficial to apply optimization similar to #126237 to it. This would allows ESQL to directly load values from a stored field without the need to synthesize the entire synthetic _source.

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Pinging @elastic/es-storage-engine (Team:StorageEngine)

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Pinging @elastic/search-eng (Team:SearchOrg)

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Pinging @elastic/search-relevance (Team:Search - Relevance)

[carlosdelest]

@ioanatia @Mikep86 , FYI