Support configurable initial/minimum heap size relative to (/as a percentage of) calculated Xmx

In the 3.x line of the buildpack, config/open_jdk_jre.yml supported a memory_initials mapping, allowing the initial size of each memory region (heap, permgen, metaspace) to be configured as a percentage of its calculated maximum. (https://github.com/cloudfoundry/java-buildpack/blob/3.x/docs/jre-open_jdk_jre.md). This option no longer appears from v4.0 onward (2017), including the current Go-based v5.x buildpack.

Because -Xmx is only known at container start (it depends on the memory limit assigned to the app instance), we cannot set a matching or proportional -Xms value ourselves via JAVA_OPTS, because we'd need to know the calculated -Xmx ahead of time. In practice this means our JVMs start with a small default initial heap and grow it under load, causing avoidable heap-resize activity that didn't happen back when memory_initials was available.

Could memory_calculator support such an option again to set the initial heap size as a percentage of the calculated max heap? This would let us keep using the automatic memory calculation while avoiding heap growth pauses after startup.

memory_calculator エントリーポイントから開始し、現在の動作を config/open_jdk_jre.yml にある 3.x の memory_initials マッピングおよび関連ドキュメントと比較します。手動で Xmx を把握しなくても、自動計算された最大ヒープの割合として初期ヒープサイズを設定できるオプションが実現すれば、作業は完了です。