fetch-content/fetch tasks should allow pulling git submodules to a configurable depth (or infinite)

[bhearsum]

In the Firefox translations project we pull a number of repositories to build toolchains from. Many of these have submodules, and some of those submodules have submodules, etc. etc. At the moment, we only end up fetching the immediate submodules when cloning, which means we end up fetching the nested submodules when we do toolchain builds.

I don't think it should be the default - but I think we should be able to instruct fetch-content to whatever depth is needed to pull in everything.