Use fetchSource API instead of fetchFields for LAST/FIRST/TAKE aggregation

[LantaoJin]

Description

Use fetchSource API instead of fetchFields for LAST/FIRST/TAKE aggregations:

fetchFields cannot be used in TEXT field pushdown, for example | stats last(name)

Related Issues

Resolves #5086

Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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