Allow insecure protocol

[markslater]

Implements #163 to allow Gradle 7.0+ to download Node over HTTP: Adds allowInsecureProtocol flag to NodeExtension taking three possible values:

1. true: Allow download over HTTP
2. false: Require download over HTTPS
3. null: Use Gradle default behaviour for permitting/denying HTTP downloads (permit prior to Gradle 7.0, deny from Gradle 7.0 on)

[markslater]

Let me know if this needs further changes, or if I've got the wrong end of the stick with any of the review comments.

[ajzawawi]

@bsautel Would greatly appreciate it if we can get this merged. We have the same issue where we're behind a corporate proxy and need this option to resolve our node dependency.

[blling]

When to release? :-)

[deepy]

It's coming in the next version, but there were test failures when I ran the tests locally so I need to look into if it was the tests or the merge broke