Add the ability to use local container configuration templates

[Colonial-Dev]

This is a riff off #4852.

It strikes me as odd that this extension only allows using container definitions from a curated list of remote repositories, especially when this list has gaping holes (for example, no Fedora images.)

Unfortunately, this makes creating and using your own templates impossible without significant hassle. I'd propose adding some way to read spec-compliant templates from disk and display them in the New dev container... definition menu. Perhaps a setting that takes an array of search paths?