[ML] Add mixed cluster tests for inference

[maxhniebergall]

Added mixed cluster tests for the inference service. I copied the integration tests from the Upgrade tests, and I copied the MixedCluster set up from this PR

To run the new mixed cluster tests

this command can be used: ./gradlew ':x-pack:plugin:inference:qa:mixed-cluster:v8.15.0#javaRestTest'. However, to run the mixed cluster tests against main (rather than a previously released version), the code base must be changed, and this flag must be added to the gradlew command -Dtests.bwc.refspec.8.15=origin/main. The code changes required to run bwc-mixed-cluster tests against main have been added to TESTING.asciidoc and are:

Another use case, since the introduction of serverless, is to test BWC against main in addition to the other released branches. To do so, specify the bwc.refspec remote and branch to use for the BWC build as origin/main. To test against main, you will also need to create a new version in [Version.java](https://github.com/elastic/elasticsearch/pull/server/src/main/java/org/elasticsearch/Version.java), increment elasticsearch in [version.properties](https://github.com/elastic/elasticsearch/pull/build-tools-internal/version.properties), and hard-code the project.version for ml-cpp in [ml/build.gradle](https://github.com/elastic/elasticsearch/pull/x-pack/plugin/ml/build.gradle).

Manual testing

I tested making changes to the serialization TextEmbeddingResults and only syntax errors were detected as failures, adding new fields didn't cause failures (I'll have to think more about whether this makes sense or not).

I removed CohereEmbeddingsTaskSettings from the InferenceNamedWriteablesProvider and this caused the CohereServiceMixedIT to fail, which is great!

I added a fake field to CohereEmbeddingServiceSettings StreamInput and the test failed. I added a new field to CohereEmbeddingServiceSettings toXContent and the test succeeded. I added a new field to CohereEmbeddingServiceSettings writeTo and the test succeeded.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[maxhniebergall]

I am confused that the concrete test classes inherit from BaseMixedIT not MixedClusterSpecIT as the MixedClusterSpecIT has the mixed cluster from Clusters.java. Should BaseMixedIT be using public static ElasticsearchCluster cluster = Clusters.mixedVersionCluster();

Thanks for calling this out! It seems that this hierarchy mixup was hiding some underlying problems.