[AI Connector] Kibana inference connector should support openai compatible schema for completion task

[YulNaumenko]

Extend Kibana AI Connector sub actions to support _unified completion task execution with the usage openai schema as a standard.

this.registerSubAction({
      name: SUB_ACTION.UNIFIED_COMPLETION,
      method: 'performApiUnifiedCompletion',
      schema: ChatCompleteParamsSchema,
    });

Elasticsearch related ticket to implement the _unified inference endpoint elastic/elasticsearch#117589

Usage example:

await actionsClient.execute({
          actionId: connectorId,
          params: {
            subAction: 'invokeAIRaw',
            subActionParams: {
              messages: prepareMessages(inputBody.messages),
              temperature: params.temperature ?? inputBody.temperature,
              stop: inputBody.stop,
              max_tokens: params.maxTokens ?? inputBody.max_tokens,
              tools: inputBody.tools,
            },
          },
        })

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[YulNaumenko]

Resolved by #200249