Machine-readable configuration output

[sunshowers]

Related:

• [meta] tell users what configuration their tests are running with #1525
• Human-readable configuration output #1526

Similar to #1526, we'd like a machine-readable way for configuration output to be presented. Some thoughts:

• Probably want options on the nextest show-config UI.
• Do we also want to let users add configuration to cargo nextest list? Seems like some tools might benefit from it.