(WIP) Adds UiRenderGraphConfig analogous to BaseRenderGraphConfig

[mtsr]

Because I added a post processing pass between Main and Swap Chain, I had a need to have UI use another Depth Texture.

This PR adds a UiRenderGraphConfig analogous to BaseRenderGraphConfig.

I haven't yet added all the options or fixed up usages throughout Bevy, because I first wanted to hear back whether this seems like a worthwhile approach.

[rmsc]

I'm developing a plugin that needs a small change to the base render graph. It turns out deviating from the default BaseRenderGraphConfig in a plugin is really awkward. I either forced the plugin user to do the modifications themselves, or I had to implement my own version of DefaultPlugins from scratch.

I proposed #1382 as a way for a plugin to modify the default configuration. The feedback I got is that BaseRenderGraphConfig and others will likely be dropped in favor of configuration resources, which I tend to favor.

That said, maybe for the short term this is the way to go.