Add support to on-demand migrate notification template content to support unicode

[darshanasbg]

Proposed changes in this pull request

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• Maintaining notification template content in unicode supported CONTENT column during the add/updates
• When retrieving, lookup on the unicode supported CONTENT column first and fallback to SUBJECT, BODY, FOOTER columns as the fallback option.

In order to enable this capability, following config should be set in the deployement.toml.

[notification_templates]
enable_unicode_support = "hybrid"

As a backup option, content also kept added/updated to the SUBJECT, BODY, FOOTER columns.. So in a case where it needed to switch from hybrid mode to the false mode, it can be done without loosing any data, apart from unicode support.

This temporal behavior makes it easier to switch to using following changes in graceful manner when the previous schema is already in use.

• Store notification content as binary to support unicode wso2/carbon-identity-framework#6120
• Store notification content as binary to support unicode #280

Related to,

• Have the ability to support non-latin unicode characters for email templates wso2/product-is#21235

When should this PR be merged

Immediate

Follow up actions

N/A

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Link: https://github.com/wso2/product-is/actions/runs/12142632257

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Approving the pull request based on the successful pr build https://github.com/wso2/product-is/actions/runs/12142632257