debug: add message `SheetDescriptor` to describe a worksheet for debugging

[wenchy]

The proto message SheetDescriptor should describe the whole worksheet details for easy locating cell (row and column):

message	worksheet
scalar field value	cell value
struct field value	cross-cell values

Refer: https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/descriptor.proto#L768