Support ETag for files

[vladisavvv]

[vladisavvv]

I have the next cases:

• Read/write/delete file with etag specified in "If-Match"
• Read with etag in "If-None-Match"
• And after any file request I would like to have access to the value of "ETag" header from the response

One more things that I have, but possible we should create a separate issue for it:

• Support some interface for http://dial/v1/ops/resource/subscribe