[blob] Support uploading base64-encoded blobs

Summary:
Address ENG-9413.

As an alternative to blob_data multipart form field, added a base64_data field which accepts base64-strings.

I could take advantage of multipart "field types" (MIME types for fields) and check type of existing blob_data but there's a risk that React Native fetch would not support it correctly, so went with a simple solution.

Test Plan: Tested on staging with D13495

Reviewers: tomek, kamil, varun, will

Reviewed By: will

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13503