helper/schema: CoreConfigSchema method for Backend
helper/schema: CoreConfigSchema method for Backend

This is just like the method of the same name on Resource, adapting the
helper/schema model of schema into Terraform Core's idea of schema.

brandontosch/terraform,pryorda/terraform,mattray/terraform,luis-silva/terraform,drebes/terraform,apparentlymart/terraform,brandontosch/terraform,iljaskevic/terraform,pradeepbhadani/terraform,brandontosch/terraform,mattray/terraform,tommynsong/terraform,luis-silva/terraform,nlamirault/terraform,luis-silva/terraform,pradeepbhadani/terraform,ewbankkit/terraform,tpounds/terraform,pryorda/terraform,mattray/terraform,youhong316/terraform,gechr/terraform,youhong316/terraform,iljaskevic/terraform,iljaskevic/terraform,hashicorp/terraform,rnaveiras/terraform,drebes/terraform,tpounds/terraform,heimweh/terraform,apparentlymart/terraform,pdecat/terraform,youhong316/terraform,ewbankkit/terraform,apparentlymart/terraform,pryorda/terraform,VladRassokhin/terraform,VladRassokhin/terraform,heimweh/terraform,tpounds/terraform,mattray/terraform,ewbankkit/terraform,luis-silva/terraform,brandontosch/terraform,brandontosch/terraform,iljaskevic/terraform,nlamirault/terraform,drebes/terraform,nlamirault/terraform,tommynsong/terraform,pdecat/terraform,pdecat/terraform,rnaveiras/terraform,pradeepbhadani/terraform,heimweh/terraform,VladRassokhin/terraform,VladRassokhin/terraform,pdecat/terraform,gechr/terraform,heimweh/terraform,ewbankkit/terraform,drebes/terraform,youhong316/terraform,tommynsong/terraform,hashicorp/terraform,apparentlymart/terraform,gechr/terraform,heimweh/terraform,rnaveiras/terraform,tpounds/terraform,hashicorp/terraform,apparentlymart/terraform