Add Metallic, Base Color to texture bake types

I'm trying to bake a game asset for a PBR workflow, and created a high poly asset using a bunch of procedural materials. From my high poly, I want to bake out the following textures to my low poly mesh:

• color
• roughness
• normal tangents
• metallic

Currently, baking out metallic is not supported, so I have to work around it using another slot which is a little annoying but works fine. However, baking out the diffuse, will also bake out the parts that are metallic as black. Technically, this makes sense as that is what metalness does, but it doesn't quite fit with how most game engines today handle their PBR texture maps, with metal elements being bright on the albedo rather than black.

So a solution here would be:

• add a baking option for 'METALLIC'
• In the diffuse Bake Type, make 'COLOR' just bake down the base color without metal darkening. Maybe adding an option called 'ALBEDO' could be useful as the bake type that does get darkened in case you still want it to do this.

Getting these changes would help a lot in moving Blender forward for being a proper PBR texture creation tool. For a large majority of modern game assets, you need 5 maps: color, roughness, normals, metalness and AO. Making sure Blender can bake all these easily would be a big step in fleshing out PBR texture creation for Blender. Eevee supports all these maps already, we just need to be able to bake them!