feat: Allow specifying alarmDedupeStringSuffix in CustomAlarmThreshold (#622)

Addresses a gap where the deduplication string wasn’t easily customizable via typical facade usage. It can be customized using dedupeStringOverride, but this overrides the entire deduplication string. Since alarm name prefixes (globalPrefix, localPrefix in AlarmNamingStrategy) are usually specified, alarmDedupeStringSuffix is more convenient in use.

I've observed many cases where dedupeStringOverride is used, with the string being
 manually constructed by concatenating prefixes passed as parameters to the facade.

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Author: slawek-amzn