Register system index descriptors through SystemIndexPlugin.getSystemIndexDescriptors

[cwperks]

Issue #, if available:

This PR registers the system indices in this plugin through the SystemIndexPlugin extension point in core. These indices will not be functionally different than they are today, its just a formal registration as a system index.

Related to: https://github.com/opensearch-project/security/issues/4439

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[cwperks]

@bowenlan-amzn Can this be merged?

[bowenlan-amzn]

@cwperks I notice there's one test failure

REPRODUCE WITH: ./gradlew ':alerting:integTest' --tests "org.opensearch.alerting.DocumentMonitorRunnerIT.test execute monitor generates alerts and findings with per alert execution for actions" -Dtests.seed=E48D87D2E1142E72 -Dtests.security.manager=false -Dtests.locale=ta-IN -Dtests.timezone=Pacific/Niue -Druntime.java=17

org.opensearch.alerting.DocumentMonitorRunnerIT > test execute monitor generates alerts and findings with per alert execution for actions FAILED
    org.opensearch.client.WarningFailureException: method [POST], host [http://[::1]:43439], URI [/_refresh?expand_wildcards=open%2Chidden], status line [HTTP/1.1 200 OK]
    Warnings: [this request accesses system indices: [.opendistro-alerting-alert-history-2024.07.02-1], but in a future major version, direct access to system indices will be prevented by default, this request accesses system indices: [.opendistro-alerting-alerts], but in a future major version, direct access to system indices will be prevented by default, this request accesses system indices: [.opensearch-notifications-config], but in a future major version, direct access to system indices will be prevented by default]
    {"_shards":{"total":15,"successful":13,"failed":0}}
        at __randomizedtesting.SeedInfo.seed([E48D87D2E1142E72:1A6999FE631E4BFA]:0)
        at app//org.opensearch.client.RestClient.convertResponse(RestClient.java:381)
        at app//org.opensearch.client.RestClient.performRequest(RestClient.java:355)
        at app//org.opensearch.client.RestClient.performRequest(RestClient.java:330)
        at app//org.opensearch.test.rest.OpenSearchRestTestCase.refreshAllIndices(OpenSearchRestTestCase.java:718)
        at app//org.opensearch.alerting.DocumentMonitorRunnerIT.test execute monitor generates alerts and findings with per alert execution for actions(DocumentMonitorRunnerIT.kt:687)

can you fix this? probably a common problem, not sure if the refreshAllIndices method needs to be updated https://github.com/opensearch-project/OpenSearch/blob/a1faafed3ad88dd8f863e40766b7f035df0ed99b/test/framework/src/main/java/org/opensearch/test/rest/OpenSearchRestTestCase.java#L706-L716