feat: extend external secrets support to mssql subchart

[Astabh]

The main Octopus chart already supports external secrets via the createSecrets flag - this PR adds the same capability to the MSSQL subchart.

Currently, users wanting to use external secrets management can do so for the main Octopus application secrets, but the MSSQL subchart always creates its own internal secret for the sa user. This creates inconsistency and prevents full external secrets management.

• Added createSecrets flag to mssql subchart for consistency with main chart.
• Added conditional secret creation in charts/mssql/templates/secret.yaml.
• Amended the SA_PASSWORD env var reference in charts/mssql/templates/statefulset.yaml to support both internal and external secrets.
• Added an init container to charts/octopus-deploy/templates/statefulset.yaml for dynamic database connection string construction when using external secret for the SA password.
• Updated charts/octopus-deploy/values.yaml with mssql external secrets configuration options and documentation.
• Added an external secrets section to README.md

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Latest commit: 2e5ec62

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