E2E: Clean config parsing

[pmikolajczyk41]

• [y] y/n | Does it introduce breaking changes? (although an extremely cosmetic one)
• [n] y/n | Is it dependant on the specific version of cargo-contract or pallet-contracts?

Description

Instead of parsing e2e configuration arguments, checking for errors and handling all situations manually, we can use darling library that is dedicated for exactly such boilerplate.

I have removed most of the testcases (leaving only the positive scenario) - they seem to be completely unnecessary now, since all the error detection and handling is now outsourced. We may however bring them back (preferably in another PR) as UI tests for e2e crate.

The only breaking change is that we won't accept runtime-only literal (only runtime_only is now valid). Additionally, we accept path arguments (like runtime) both in raw form and as literals (default in darling).

Note: we had to change additional_contracts field type from Vec<String> to String, since darling doesn't support parsing Vec<String> in the intended way (only by multiple argument occurrences). However, this is not visible from outside struct internal implementation.

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have added an entry to CHANGELOG.md
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules

[ascjones]

Love it ❤️