[JENKINS-75014] Allow auth module to see sts module classes

[basil]

Before this PR, I was seeing https://github.com/aws/aws-sdk-java-v2/blob/84b5a6cb7530eb332dec50581d8f490a23ca4531/core/auth/src/main/java/software/amazon/awssdk/auth/credentials/internal/WebIdentityCredentialsUtils.java#L49 which took me to https://github.com/aws/aws-sdk-java-v2/blob/84b5a6cb7530eb332dec50581d8f490a23ca4531/utils/src/main/java/software/amazon/awssdk/utils/ClassLoaderHelper.java#L124, showing that this code needs SetContextClassLoader to work with the Jenkins plugin class loading system.

Testing done

After this PR, could no longer reproduce the exception described in the issue description.

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