metabook: add API to return all workbooks’ descriptors

[wenchy]

Note: if this workbook has no metasheet @TABLEAU, then auto scan and find the main workbook (with Merger configured).

• [ ] use Protobuf to define returned data, like google/protobuf/descriptor.proto.

Tableau Descriptor

add new file descriptor.proto:

message BookDescriptorSet {
  map<string, BookDescriptor> books = 1;
}
// Describes a workbook file.
message BookDescriptor {
  string name = 1;
  WorkbookOptions book = 2;
  map<string, SheetDescriptor> sheets= 3;
}
// Describes a worksheet message.
message SheetDescriptor {
  string name = 1;
  WorksheetOptions sheet = 2;
}