[ML] Use registry for transform templates

[davidkyle]

Change the way transform templates are installed to use a IndexTemplateRegistry with mappings stored in resource files.

This is the typical way of doing things for many plugins and is the same approach used by ml. This change will enable some common code changes to the auditor in transforms and ml.

[davidkyle]

This test is key because it proves the mappings have not been altered by the refactoring. The above test asserts none of the other settings have changed.

The mappings defined in TransformInternalIndex are used in this test and no longer required once I've proved the mappings have not changed. Once the build is green and the PR approved I'd like to remove the static mapping definitions in TransformInternalIndex and this test.

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)

[davidkyle]

The build failure is from TransformAuditorIT.testAliasCreatedforBWCIndexes

@hendrikmuhs I cannot see how this change would break that test, could you take a look please

java.lang.AssertionError
	at __randomizedtesting.SeedInfo.seed([9EAF9B8CC387C078:CC370CA406B841BF]:0)
	at org.junit.Assert.fail(Assert.java:86)
	at org.junit.Assert.assertTrue(Assert.java:41)
	at org.junit.Assert.assertTrue(Assert.java:52)
	at org.elasticsearch.xpack.transform.integration.TransformAuditorIT.lambda$testAliasCreatedforBWCIndexes$2(TransformAuditorIT.java:110)
	at org.elasticsearch.test.ESTestCase.assertBusy(ESTestCase.java:934)
	at org.elasticsearch.test.ESTestCase.assertBusy(ESTestCase.java:907)
	at org.elasticsearch.xpack.transform.integration.TransformAuditorIT.testAliasCreatedforBWCIndexes(TransformAuditorIT.java:109)

[davidkyle]

The test failure is fixed by #63162

[davidkyle]

Tests have shown that the use of the template registry rather than the Plugin::getIndexTemplateMetadataUpgrader() method causes some problems because the registry updates the templates sometime after the master node comes up.
In the case of rolling upgrades this is not soon enough.

We could add logic to wait for the templates to be installed but the extra complications and hard to maintain code are not worth the small simplification using the registry would enable.

Closing without merging