This commit reworks BootRun so that it no longer subclasses JavaExec.
This provides Boot with greater control of how the executed JVM is
configured, including the possibility of using @Option to provide args
and JVM args via the command line (gh-1176). It also allows some usage
of convention mappings to be removed in favour of PropertyState and
Provider (gh-9891). For users who relied up the advanced (and rather
complex) configuration options provided by JavaExec, an escape hatch
is provided by allowing the JavaExecSpec that's used to execute the
JVM to be customized.

Closes gh-9884