feat(chart): add database name configuration

[millin]

This PR allows configuring the database name through Helm values. Previously, the database name was hardcoded in the templates.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Docs related to the changes have been added/updated (if applicable)

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Note

Low Risk
Small Helm chart change that only parameterizes the database name used by setup jobs; low risk aside from potential misconfiguration leading to jobs targeting the wrong DB.

Overview
Adds a new Helm value global.sql.datasource.database (default datahub) and wires it into the MySQL and PostgreSQL setup job templates so DATAHUB_DB_NAME is no longer hardcoded.

Updates values.schema.json, values.yaml, and the chart README.md values table to document and validate the new database-name setting.

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