Added extensibility point to provide custom fetch implementation

[tomasherceg]

I am not sure if it is a good idea to provide this extensibility point in the public API, but the attacker can replace window.fetch anyway so it's probably not such a big deal.
Alternatively, we can bundle a special version of DotVVM for the MAUI hosted version - what do you think?

[exyi]

I'd vote for bundling special version for the MAUI, I'd guess this might not be the last thing needed there. I don't really like adding features which will never be needed in most apps and this unnecessarily increasing the bundle size (although this isn't a large change, so whatever)

[tomasherceg]

I've done this as part of this draft PR #1368.

Depends on #1329 which should be merged first.

I've put there only the necessary things for DotVVM app to run inside WebView - the other things are still in the external repo but I plan to integrate it into DotVVM repository soon.