refactor: convert to postgres values directly from arrow

[MichaelScofield]

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What's changed and what's your intention?

following #7096
for postgres

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[Copilot]

Pull Request Overview

This PR refactors the PostgreSQL type encoding logic to convert data directly from Arrow arrays instead of going through intermediate Value representations. This follows a similar optimization made in PR #7096.

Key Changes

• Replaced value-based encoding with direct Arrow array conversion for better performance
• Introduced RecordBatchRowIterator to iterate over record batches and encode rows directly from Arrow arrays
• Eliminated intermediate Value conversions by reading directly from typed Arrow arrays

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 7 comments.

File	Description
src/servers/src/postgres/types.rs	Refactored encoding functions to work directly with Arrow arrays; added RecordBatchRowIterator for row-by-row encoding; updated tests to use Arrow schema and vectors
src/servers/src/postgres/handler.rs	Simplified stream processing to use new RecordBatchRowIterator instead of manual row encoding

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[sunng87]

LGTM. And I think next step we can switch to my datafusion-postgres ecosystem library arrow-pg

[MichaelScofield]

@killme2008 PTAL