Previously supported custom parameters not passing validation

[alangsto]

#392 introduced a breaking change that prevents previously allowed custom parameters from passing validation.

Example of custom parameter that now does not pass validation:

["next=something?repo=https://github.com/org/repo?folder=/home/user&branch=main"]

Changes to the custom parameter validation should be backwards compatible, and account for cases like this where the user passes in a single string.

[alangsto]

@kuipumu tagging you as the author of #392.

[kuipumu]

@alangsto I created a PR that should address this issue #412, this should be loose enough to allow any kind on key-pair value has a custom parameter.