Feature: `manifest-array-key` for TOML plugins

[JonahPlusPlus]

I'm thinking about making a plugin for ChromeDriver, to support headless browser testing.
A TOML plugin would be very simple, but the only issue is resolving the versions available.
There is https://googlechromelabs.github.io/chrome-for-testing/known-good-versions.json, which provides all the "good" versions available, but the array of versions is nested under the key "versions".

If there was a way to look inside the array, then this plugin would be easy to add.
A manifest-array-key would fix this and allow indexing a JSON manifest composed of an object containing an array of objects.

e.g. for known-good-versions.json:

[resolve]
manifest-url = "https://googlechromelabs.github.io/chrome-for-testing/known-good-versions.json"
manifest-array-key = "versions"
manifest-version-key = "version"