Add 'blob' record upload endpoint

[pinkwah]

In addition to POST /ensembles/../records/../file, we should have a POST /ensembles/../records/../blob.

The file endpoint uses standard HTTP file uploading scheme by using the multipart/form-data. The user uploads a file named file, as if by using an HTML form like:

<input type="file" name="file" />

This has the benefit of being usable from Swagger, where it creates such an entry, but it has drawbacks, the largest of which is that it seems that FastAPI consumes the entire stream before the endpoint function is invoked. This leads to a substantial waiting time while the file is being uploaded to FastAPI, before it gets forwarded to blob storage.

The /blob endpoint, however, uses Starlette's Request class, which is represents the raw request. One of its functions is async def stream(self) -> AsyncGenerator[bytes, None]), which reads the stream in chunks, and allows us to send it further, without having to load the entire document as an intermediate step.

This is currently blocked by Azure Blob Storage not supporting async data uploads: Azure/azure-sdk-for-python#17478

[mortalisk]

We have an endpoint for this. It is a bit unclear what this issue is about. Particularly the title. Could you clarify a bit @pinkwah ?