Provide support for In-memory template manager

[JeethJJ]

Recreating PR : #254

Describe the issue:
When introducing new notification templates (Email/SMS) and migrating customers to on-premises solutions, a data migration is currently required for existing tenants. However, with the removal of data migration support in upcoming Identity Server (IS) releases, we need to address this issue to ensure smooth template integration without relying on data migration.

Proposed changes in this pull request
Currently email/sms templates are stored in the registry, for any sub tenants we are not adding new templates when server startup. When creating any tenants we are adding default email templates to the registry per tenant, this leaves redundant data. With this fix, we are providing In-memory template manager which return default templates and act on read only mode.

Currently, it works with hybrid template manager and db-based template manager also.

Issue : wso2/product-is#20996

[jenkins-is-staging]

PR builder started
Link: https://github.com/wso2/product-is/actions/runs/10573242101

[jenkins-is-staging]

PR builder completed
Link: https://github.com/wso2/product-is/actions/runs/10573242101
Status: failure

[jenkins-is-staging]

PR builder started
Link: https://github.com/wso2/product-is/actions/runs/10577681037

[jenkins-is-staging]

PR builder completed
Link: https://github.com/wso2/product-is/actions/runs/10577681037
Status: success

[jenkins-is-staging]

Approving the pull request based on the successful pr build https://github.com/wso2/product-is/actions/runs/10577681037

[jenkins-is-staging]

PR builder started
Link: https://github.com/wso2/product-is/actions/runs/10608829457

[jenkins-is-staging]

PR builder completed
Link: https://github.com/wso2/product-is/actions/runs/10608829457
Status: success

[jenkins-is-staging]

Approving the pull request based on the successful pr build https://github.com/wso2/product-is/actions/runs/10608829457

[darshanasbg]

Also with this change we dont need to persist default templates with server startup[1] and tenant creation[2].. So lets remove those and associate code that become unused with that..

In fact we can,

• Remove whole TenantManagementListener.
• Remove loadDefaultEmailTemplates() loadDefaultSMSTemplates() method impls
• Have EmailTemplateManager.addDefaultEmailTemplates() method deprecated
• Have EmailTemplateManagerImpl.addDefaultEmailTemplates() method deprecated & clean the impl to throw not implemented exception.
• Have NotificationTemplateManager.addDefaultNotificationTemplates() method deprecated
• Have EmailTemplateManagerImpl.addDefaultNotificationTemplates() method deprecated & clean the impl to throw not implemented exception.

[1]

identity-event-handler-notification/components/email-mgt/org.wso2.carbon.email.mgt/src/main/java/org/wso2/carbon/email/mgt/internal/I18nMgtServiceComponent.java

Lines 144 to 146 in d610d0c

	// Load default notification templates.
	loadDefaultEmailTemplates();
	loadDefaultSMSTemplates();

[2]

identity-event-handler-notification/components/email-mgt/org.wso2.carbon.email.mgt/src/main/java/org/wso2/carbon/email/mgt/internal/TenantManagementListener.java

Lines 48 to 56 in d610d0c

	templateManager.addDefaultEmailTemplates(tenantDomain);
	} catch (I18nEmailMgtException e) {
	String message = "Error occurred while loading default email templates for the tenant : " + tenantDomain;
	log.error(message);
	throw new StratosException(message, e);
	}
	try {
	NotificationTemplateManager notificationTemplateManager = new EmailTemplateManagerImpl();
	notificationTemplateManager.addDefaultNotificationTemplates(

[jenkins-is-staging]

PR builder started
Link: https://github.com/wso2/product-is/actions/runs/10612123038

[JeethJJ]

Changes suggested through [1] will be addressed in another PR.

[1] #256 (comment)

[jenkins-is-staging]

PR builder completed
Link: https://github.com/wso2/product-is/actions/runs/10612123038
Status: cancelled

[jenkins-is-staging]

PR builder started
Link: https://github.com/wso2/product-is/actions/runs/10612717489

[jenkins-is-staging]

PR builder completed
Link: https://github.com/wso2/product-is/actions/runs/10612717489
Status: cancelled

[jenkins-is-staging]

PR builder started
Link: https://github.com/wso2/product-is/actions/runs/10612848361

[jenkins-is-staging]

PR builder completed
Link: https://github.com/wso2/product-is/actions/runs/10612848361
Status: success

[jenkins-is-staging]

Approving the pull request based on the successful pr build https://github.com/wso2/product-is/actions/runs/10612848361