[CASSANDRA-21093] Custom startup checks plugin

[nickbar01234]

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The Cassandra Jira

[michaelsembwever]

question: what's the value in exposing this method ?

is it solely about order versus calls to withTest(StartupCheck)

suggestion: rename for added intuition

Suggested change

	public StartupChecks withExternalTests() {
	public StartupChecks withServiceLoaderTests() {