Refactor duplicate JRE implementations into shared BaseJRE struct

[stokpop]

Currently the Go buildpack has 6 nearly identical JRE implementations (~1300 lines total): OpenJDK, SapMachine, Zulu, IBM, Oracle, GraalVM. Each has its own copy of Supply(), Finalize(), MemoryCalculatorCommand() etc. with only names/strings differing.

In the Ruby buildpack, this was solved cleanly via inheritance — e.g. ZuluJRE < OpenJDKLike was 3 lines.

Proposed fix

Introduce a shared BaseJRE struct with Go embedding:

type BaseJRE struct {
    ctx        *common.Context
    jreName    string
    envVarName string
    memoryCalc *MemoryCalculator
    ...
}

// Supply(), Finalize(), MemoryCalculatorCommand() implemented once on BaseJRE

type ZuluJRE struct { BaseJRE }
type SapMachineJRE struct { BaseJRE }
// etc.

This would reduce ~1300 lines to ~200 lines of shared logic + ~5 lines per vendor.

Context

Discovered while fixing #1257 — the LoadConfig() bug (reading only JBP_CONFIG_OPEN_JDK_JRE for all JREs) was a direct consequence of the duplicated structure. A minimal fix was applied to pass jreName to NewMemoryCalculator. The base struct refactor is the structural follow-up.