Clients will query for blobs flagged PENDING so they can start
processing. This commit adds flagging, but still in a hacky way as the
backend requires the flags as a json string inside a request. A refactor
to separate request handling from the backend itself will solve it in
the near future.

-- Related: #8874

Improves error handling, add missing header to __init__.py and remove
mkdirs from flags methods. This is a commit from code review on !117.

Improve test naming, creates a "count" method and filter flags files
properly.

filter_flag parameter can now be used to filter listing blobs by a
specific flags. Eg: I can ask for blobs on incoming namespace flagged as pending.

-- Resolves: #8913

By using `only_count=True` parameter, instead of returning a full list,
server just count the amount of blobs.

-- Resolves: #8871

IncomingBox spec has a flags feature for the processing flow of
messages. This commit adds it using a .flags file.

-- Resolves: #8869

Refactor suggested from !105 review.