Add base64_encoded_blob field

What does this MR do and why?

Add a graphql field to return blobs encoded base64.

Currently blobs are encoded UTF-8 when served via GraphQL. Here we serve the raw blob encoded base64 to maintain their integrity. This solves a problem where non UTF-8 encoded blobs can become malformed.

See #388196 for more information.

This MR is an iteration on a previous attempt at resolving this issue by escaping incompatible characters. That solution was found not to be suitable. I have removed references to that and updated with the new solution in this MR.

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Related to #388196