Introduces a new `oneof` arm on `TrainerResourceConfig` /
`InferencerResourceConfig` that lets callers describe a launcher as a
shell command + positional args, instead of a fixed-shape Vertex AI /
KFP / local resource config. The proto carries no semantics here — the
dispatcher is added in a follow-up PR; this commit only ships the
message, regenerated bindings, and the wrapper-property update so
downstream code can read `wrapper.trainer_config` and get a
`CustomResourceConfig` back.

The diff includes a long tail of cosmetic Scala changes outside
`gigl_resource_config/` because scalapbc regenerates every sibling
proto's emitted source whenever any one proto in the same directory
changes. Reviewers can scope to `CustomResourceConfig.scala` and the
`*ResourceConfig.scala` siblings that gain the new oneof case.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CustomResourceConfig is launcher-pluggable — there is no concrete
machine spec to validate against (no machine_type, num_workers, GPU
config, etc.). The wrapper's trainer_config / inferencer_config
properties now return a union that includes CustomResourceConfig
(introduced earlier in this PR), but _validate_machine_config does
not accept it.

Add an isinstance early-return guard at each call site that logs
the skip and returns. Shape and backend-compatibility validation
for CustomResourceConfig come in a follow-up PR; this commit only
makes the existing validation flow type-clean against the widened
union.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Implements `launch_custom`, a thin shim that takes a populated
`CustomResourceConfig` and shells out via
`subprocess.run(shell=True, check=True)`. The proto's `command` is
a shell snippet (so leading `KEY=VALUE` env assignments parse
naturally) and `args[]` are individually `shlex.quote`-d before
joining, so values containing whitespace survive the shell pass.

The dispatcher performs no template substitution: `command` and
`args[]` are taken verbatim, and any placeholder text reaches
`subprocess.run` literally. Consumers that want runtime-context
substitution (e.g. ${gigl:foo}) should resolve it at YAML-load
time before the proto reaches this module. No call site in the
rest of the repo invokes `launch_custom` yet — wiring is added in
a follow-up PR.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>