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Honor the configured code system when adding FHIR code translations

FhirR4.addTranslation took a code system, used it to look up a CodeMapper,
and then always emitted the ICD-10-CM system URI. It also only ever asked
for the ICD-10-CM mapper, so a map registered under any other
exporter.code_map.<system> key was loaded at startup and never consulted,
even though Exporter#loadCodeMappers documents that usage and gives CPT as
its example.

Apply every configured code map and label each translation with its own
system URI. Output for the shipped ICD-10-CM configuration is unchanged.

Also add the CPT system URI, match system names case-insensitively so that
names derived from configuration keys resolve, and hold the code mappers in
a TreeMap so that multiple maps are applied in a deterministic order.