Add FixedString to PostgreSQL mapping

[pamarcos]

Add FixedString to PostgreSQL mapping

Summary

This should be merged only after ClickHouse/ClickHouse#77304

Checklist

• Delete items not relevant to your PR
• URL changes should add a redirect to the old URL via https://github.com/ClickHouse/clickhouse-docs/blob/main/docusaurus.config.js
• If adding a new integration page, also add an entry to the integrations list here: https://github.com/ClickHouse/clickhouse-docs/blob/main/docs/integrations/index.mdx