This change adds the `rbt post` logic for uploading binary files to diffs. We start by assembling a list of the newly-created FileDiffs that are marked as binary (using the new binary flag to the FileDiff API). This is either done for the individual commits (when posting with commit history) or for the full diff (when posting a squashed diff or on tools that do not support history). We then go through and find what files we should upload, filtering by whether it's a reviewable MIME type and the configured file limit. We then create new file attachments for each matching binary file. This includes the base SCMClient flag and method definitions, but does not implement upload for any SCMs. We have to do it per client because we need to add new APIs to the clients to get file content (and optionally file size) at specific revisions. Individual per-SCM implementations will come in future changes. Testing Done: - Ran unit tests. - Posted a whole bunch of changes with Git and SVN repositories that included image files. Tested with both commit history and squashed diffs.