bootctl: expose --esp-path/--boot-path/--make-entry-directory via Varlink (#42838)

The Install/Unlink/Link/LinkAuto Varlink methods always auto-discover
the ESP and XBOOTLDR partitions and Install always runs
make-entry-directory in auto mode, so IPC callers cannot match what the
CLI verbs do with `--esp-path`, `--boot-path` and
`--make-entry-directory`. Add optional `espPath`, `xbootldrPath` and
`makeEntryDirectory` parameters that feed into the same code paths the
CLI already uses.