Add support for `--example` to `cargo chef cook`

[AnomalRoil]

I'm building some example code using:

FROM chef AS planner

COPY . .
RUN cargo chef prepare --recipe-path recipe.json --bin my-example

FROM chef AS builder

COPY --from=planner /app/recipe.json recipe.json
RUN cargo chef cook --release --recipe-path recipe.json

COPY . .
RUN cargo build --release -p my-example --example my-example

And sadly it seems the dependencies that are being compiled by the cook step are not the same ones as the ones used by cargo build --example my-example, and so it's rebuilding it all.

I've found that cook supports --examples, but it seems sometimes it's not always building all the required dependencies for my example binaries, it allows to not rebuild all the dependencies, however, only a few.

[LukeMathWalker]

You need to pass exactly the same flag to cargo build and cargo chef cook.
To make that possible, we're missing one option at the moment in cargo chef cook, --example.