Implement a more efficient way to handle closure operation requests that have more than one coding

[JohnTimm]

One step towards a more efficient way to handle more than one coding in a closure operation request is to add a closure method signature to the FHIRTermServiceProvider SPI that supports multiple codes. For example:

Map<Code, Set<Concept>> closure(CodeSystem codeSystem, Set<Code> codes);

and a closure method signature to FHIRTermService:

Map<Coding, Set<Concept>> closure(Set<Coding> codings);

See: https://chat.fhir.org/#narrow/stream/179202-terminology/topic/ibm.20as.20a.20fhir.20terminology.20server for more info

[lmsurpre]

I confirm that the new closure variants (closure(Set<Coding> codings) and closure(CodeSystem codeSystem, Set<Code> codes)) have been added to FHIRTermService.
closure(Set<Coding> codings) is covered by a unit test and is now used by the ClosureOperation.

closure(CodeSystem codeSystem, Set<Code> codes) doesn't seem to be covered by any tests, but its a one-line implementation the delegates to closure(CodeSystem codeSystem, Set<Code> codes) and I confirmed that both our registry-based and graph-based implementations for that are covered by FHIRTermServiceProviderTest.testClosure2().

One minor thing I noticed is that one variant of the closure method (Set<Concept> closure(CodeSystem codeSystem, Code code)) is missing javadoc, so that might be good to add.

[lmsurpre]

the javadoc comment is now added