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.

Start at the memory_calculator entry point and compare its current behavior with the 3.x config/open_jdk_jre.yml memory_initials mapping and linked documentation. The work is done when an option can set initial heap size as a percentage of the automatically calculated maximum heap without requiring a manually known Xmx.