Support table DDLs in the information schema for common OLAPs

[begelundmuller]

This PR adds support for fetching view and table DDLs in information schema queries for OLAP drivers. It also propagates this DDL to the show_table MCP tool call for use by the developer agent.

It implements DDL support for the following drivers:

• BigQuery
• Clickhouse
• DuckDB
• MySQL
• Postgres
• Snowflake
• Starrocks

Closes https://linear.app/rilldata/issue/PLAT-401/include-original-data-types-and-ddl-in-show-table-tool

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!