Feat/proto custom type

[rotemtam]

In prep for ent/ent#1350, adding some flexibilty for users to modify the type mapping between ent and pb.

[a8m]

Looks good, but remove the Override string from this function.

Also, consider adding a usage example in the godoc. I use it a lot.

Suggested change

	func OverrideType(typ descriptorpb.FieldDescriptorProto_Type) FieldOption {
	return func(p *pbfield) {
	p.OverrideType = typ
	}
	}
	// OverrideTypeName sets the pb descriptors type name, needed if the OverrideType attribute is TYPE_ENUM or TYPE_MESSAGE.
	func OverrideTypeName(n string) FieldOption {
	return func(p *pbfield) {
	p.OverrideTypeName = n
	}
	}
	func Type(typ descriptorpb.FieldDescriptorProto_Type) FieldOption {
	return func(p *pbfield) {
	p.Type = typ
	}
	}
	// TypeName sets the pb descriptors type name, needed if the OverrideType attribute is TYPE_ENUM or TYPE_MESSAGE.
	func TypeName(n string) FieldOption {
	return func(p *pbfield) {
	p.TypeName = n
	}
	}