Dynamic RBE Toolchain Bootstrapper (#2635)

- Discards deprecated GCS bucket pre-generation and implements
on-the-fly compilation of rbe_configs_gen via Docker-out-of-Docker
(DooD) sibling container.
- Introduces the rules/rbe_presets.json dynamic presets mapping on the
master branch, providing zero-configuration upgrades without changing
rule pins.
- Provides the rbe_config repository rule/macro and rbe_config_extension
module extension.