Add PostgreSQL dialect support for DUMP statements

[apstndb]

Current State

DUMP statements (DUMP DATABASE, DUMP SCHEMA, DUMP TABLES) currently return an error for PostgreSQL dialect databases with the message:

"DUMP statements are not yet supported for PostgreSQL dialect databases"

This was implemented in PR #434 to prevent generating incorrect SQL.

Problem

The SQL export format depends on spanvalue.LiteralFormatConfig which generates Google Standard SQL literals, not PostgreSQL-compatible ones. This causes several issues:

1. Literal formatting: SQL literals are formatted for Google SQL syntax, which may not be valid PostgreSQL
2. Identifier quoting: Would need double quotes for PostgreSQL vs backticks for Google SQL
3. Schema handling: PostgreSQL uses 'public' schema vs empty string for Google SQL

Proposed Solution

To properly support PostgreSQL dialect, we need:

1. Dialect-aware literal formatting: Either enhance spanvalue to support PostgreSQL literals or implement a separate formatter
2. Identifier quoting function: Already drafted but not used due to literal formatting limitation
3. Schema detection: Use 'public' for PostgreSQL, empty string for Google SQL

Implementation Notes

• The infrastructure for dialect detection is already in place (session.systemVariables.DatabaseDialect)
• The main blocker is the literal formatting which is deeply integrated with spanvalue
• This might require significant changes to the formatting layer

Related

• PR feat: implement DUMP statements for database export #434 - Initial DUMP implementation with explicit PostgreSQL error
• Issue Implement table dependency resolver for INTERLEAVE and FK relationships #426 - Foreign key support (separate concern)

Workaround

Users with PostgreSQL databases can use alternative export methods until this is implemented.