allow to set doppelganger bins to use

[pepoviola]

TLDR: we need to allow to set the bin (path) for running doppelganger/doppelganger-parachain to allow to embed this library in another tool (e.g EM).

What changes

Library (zombie-bite-core)

• New config::DoppelgangerSetup { relay, para }, both
  Option<String>.
  • relay_command() and para_command() fall back to
    doppelganger / doppelganger-parachain, the same defaults as before.
  • validate() checks each set value with zombienet's Command (no
    whitespace, the same rule as SpawnSetup). A value containing / must
    also be an existing, executable file. A bare name is left to PATH, like
    the default.
• New BiteOptions::doppelganger: DoppelgangerSetup field.
• bite uses it. It calls opts.doppelganger.validate() before
  anything else, so a bad path fails right away rather than when the first
  sync node can't be spawned. All four call sites then use the configured
  commands. The start-of-bite log line names the binaries in use.
• Config file: an optional [doppelganger] table with relay and
  parachain keys (config::DoppelgangerConfig), read with
  ZombieBiteConfig::get_doppelganger_setup().
• resolve_bite_config fills opts.doppelganger from the config file,
  then from two new BiteOverrides fields, doppelganger and
  doppelganger_parachain. Each flag wins over its own config key only, and
  the result is validated along with SpawnSetup.
• Removed run_doppelganger_node, which was unused and ran a hardcoded
  doppelganger --chain kusama.

CLI (zombie-bite)

• bite gains --doppelganger <path> and --doppelganger-parachain <path>.
• Commands now carries #[allow(clippy::large_enum_variant)]. The two new
  fields pushed Bite over the lint's size threshold. The enum is parsed once
  from argv, so boxing clap fields wouldn't buy anything.

Docs

• Root README:
  • the requirements now say the binaries can come from PATH or be
    configured;
  • a new "Use a specific doppelganger build" section covers the table, the
    flags, precedence, validation and keeping the workers next to the binaries;
  • the command/image section no longer says the bite "always" runs the
    PATH binaries.
• zombie-bite-core README: explains that BiteOptions::doppelganger
  and SpawnSetup are how the binaries get configured.
• New examples/doppelganger.toml.

Compatibility

• Behaviour is unchanged by default. With no flag and no
  [doppelganger] table, bite runs exactly the commands it ran before.
  Existing config files parse unchanged, because the table is optional.