Optimize significant_text aggregation to only parse the field it requires from _source

[benwtrent]

Previously created optimizations:

• Enabling XContentParser filtering for specific fields: Memory efficient xcontent filtering #77154
• Retrieving only specific fields from _source: [ML] Optimize source extraction for categorize_text aggregation #79099

Allow significant_text to only parse out the specific field it requires from _source. This allows for a minor optimization when _source is small, but can be significant (pun intended) when _source contains many fields, or other larger fields that significant_text doesn't care about.

Since significant_text does not allow sub-aggs, not caching the parsed _source is reasonable.

[elasticmachine]

Pinging @elastic/es-analytics-geo (Team:Analytics)