`org.keycloak.admin.client.resource.ClientsResource.findByClientId(String)` should return single item

### Description

Currently `org.keycloak.admin.client.resource.ClientsResource.findByClientId(String)` returns a `List` but client id is unique so it should return a single `ClientRepresentation` (nullable or `Optional`).

Now clients of the method are forced to get the first element of the collection.

### Discussion

_No response_

### Motivation

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### Details

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