use Extension to control the targets container, so that Gradle will generate Kotlin DSL accessors

[aSemy]

Improve the DSL available in build scripts.

• ⚠️ breaking change - Remove adding each target to the BCV extension, replace with targets.<nameOfTarget>
• change BCVProjectExtension to be an abstract class so it's easier to add extensions
• change targets to be backed by a Gradle extension, so Gradle generates accessors