[BUG] No restriction for creating smtp email sender with existing sender name

[zhongnansu]

Describe the bug Even thought the label of email sender name field says "Use a unique, descriptive name". There's actually no restriction in the backend to gurantee uniqueness. This will lead to secuirty risk as in the backend, the credentials are stored and retrieved by the sender name

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2. Click on '....'
3. Scroll down to '....'
4. See error

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Additional context Can refer to Alerting implementation for a fix https://github.com/opensearch-project/alerting/blob/1.0/alerting/src/main/kotlin/org/opensearch/alerting/transport/TransportIndexEmailAccountAction.kt#L176