Provide field value tracking, persisted to disk on each search_after page.

Adds `:last_value` and `:present` placeholders, allowing the plugin to inject the cursor value and now-30 seconds, respectively, in the query string.

Useful to track new data being written to an index or series of indices.

Works best with nano second precision timestamps added by Elasticsearch's Ingest Pipelines.

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Co-authored-by: Joel Andritsch <joel.andritsch@gmail.com>
Co-authored-by: Rob Bavey <rob.bavey@elastic.co>
Co-authored-by: Karen Metts <35154725+karenzone@users.noreply.github.com>