Custom Values: add proper tests of the interface

[jsdw]

In #1117 we add a statically generated interface for custom values. We test that the codegen doesn't blow up, but should also test that generated interface itself works.

Probably we want to

1. write a script that can spit out metadata that contains a bunch of custom values in (the custom values are fully contained in the metadata, so it doesn't matter about the rest of this metadata; it can just be empty).
2. Use the subxt macro pointed at this artifacts/metadata_with_custom_values.scale to generate an interface in some test file
3. Try fetching values via static and dynamic interface to check that it all works as hoped.

(1) is a bit tricky; currently we have a scripts/artifacts.sh file to generate them. I'd lean towards adding a small testing/generate_custom_metadata binary crate that can spit it out, and then having this script call cargo run on it eg cargo run --bin testing-generate-custom-metadata > artifacts/metadata_with_custom_values.scale.