make credential optional in mcp connectors

[jiapingzeng]

Description

At the moment, MCP connector requires a credentials field. However, credentials is not always needed for remote MCP servers. This PR will make it optional.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[jiapingzeng]

Test failures don't seem related to my change:

Tests with failures:
 - org.opensearch.ml.utils.IndexUtilsTests.testGetNumberOfDocumentsInIndex_SearchQuery
 - org.opensearch.ml.utils.IndexUtilsTests.testGetIndexHealth_Alias
 - org.opensearch.ml.utils.IndexUtilsTests.testGetIndexHealth_Index

Tests with failures:
 - org.opensearch.ml.action.execute.ExecuteITTests.testExecuteLocalSampleCalculator
 - org.opensearch.ml.action.prediction.PredictionITTests.testPredictionWithDataFrame_LinearRegression
 - org.opensearch.ml.action.prediction.PredictionITTests.testPredictionWithoutDataset_KMeans
 - org.opensearch.ml.action.prediction.PredictionITTests.testPredictionWithDataFrame_FitRCF
 - org.opensearch.ml.action.prediction.PredictionITTests.testPredictionWithSearchInput_LogisticRegression
 - org.opensearch.ml.action.prediction.PredictionITTests.testPredictionWithDataFrame_BatchRCF
 - org.opensearch.ml.action.trainpredict.TrainAndPredictITTests.testTrainAndPredict_BatchRCF
 - org.opensearch.ml.action.trainpredict.TrainAndPredictITTests.testTrainAndPredictWithEmptyDataset_KMeans
 - org.opensearch.ml.utils.IndexUtilsTests.testGetIndexHealth_NoIndex
 - org.opensearch.ml.action.models.SearchModelITTests.test_all