Move common methods that plugins use in ODFERestTestCase to core's OpenSearchRestTestCase

[cwperks]

There is an issue in the common-utils repo (opensearch-project/common-utils#97) to move this much-copied file from multiple plugin repos into common-utils. I am opening an issue in the core repo to discuss adding the common methods directly into the core repo in a generic way that is not tied with the security plugin. Many of the methods that are copied between plugins have to do with setting up a REST Client to make rest requests against a cluster with the security plugin installed and only accepting requests using HTTPS.

When OpenSearch uses HTTPS, REST Clients must be configured with a truststore containing the root certificate or be configured to not validate SSL (equivalent of curl's --insecure). From looking at a few plugin repos, it looks like the latter overly trustworthy approach is the most commonly used.

Potential methods that could be abstracted into core (taking methods from k-NN as an example):

• configureHttpsClient
• isHttps that uses a system property to determine to build a client for https: https://github.com/opensearch-project/k-NN/blob/main/src/testFixtures/java/org/opensearch/knn/ODFERestTestCase.java#L82-L97
• Methods to interact with system indices from AD? https://github.com/opensearch-project/anomaly-detection/blob/main/src/test/java/org/opensearch/ad/ODFERestTestCase.java#L111-L124