Add support for cargo-args in package metadata

[jyn514]

Closes #1353

[Nemo157]

Maybe mention that these will be passed to multiple subcommands so need to be generally applicable.

[jyn514]

Is that true? I thought we only ever use the rustdoc subcommand.

I can add a disclaimer that we don't guarantee the subcommand and you should make these independent, but I don't know how useful that is in practice - they can't truly be independent because e.g. cargo new won't have any useful options, and I think almost everything else is shared between rustdoc and doc.

[Nemo157]

We use cargo metadata too I think.

[jyn514]

Yes, but we don't pass user-defined args there:

docs.rs/src/utils/cargo_metadata.rs

Lines 17 to 21 in 20aa706

	let res = Command::new(workspace, toolchain.cargo())
	.args(&["metadata", "--format-version", "1"])
	.cd(source_dir)
	.log_output(false)
	.run_capture()?;

[jyn514]

If it helps, the only place we call cargo_args() is for prepare_command:

docs.rs/src/docbuilder/rustwide_builder.rs

Line 588 in 20aa706

let cargo_args = metadata.cargo_args(&cargo_args, &rustdoc_flags_extras);

which is only used for doc coverage and actually generating the documentation.