Add blob support in CBL : .Net

Replace _attachments key with blob in save documents which is done for C to use attachments on CBL

https://github.com/couchbaselabs/mobile-testapps/commit/ab3a8734d2db17e83e2675199604a53752ac1632#diff-bbed6e28fcc85a5ae920bd7bdeb38bd0b30bb1b4417b3d0c4eb18ccc0a2092a7R171

looks like we have create blob in our blobRequest handler, we can use that logic in database_saveDocuments method, we can update the logic similiar to C-platform to avoid update on all python tests for _attachment