Decode streaming data

[scotts]

🚀 The feature

We have had several requests to be able to decode streaming data, instead of requiring that the entire file be present. Upon creating a VideoDecoder, users would have some way to indicate that the file they are decoding will be streamed rather than entirely present on the file system.

Motivation, pitch

The main motivation is performance, which can come in two ways:

1. Overlapping decoding with copying the data from an external system.
2. Avoiding sending the bytes that are never used. That is, if we sample 10 frames from a 100 GB file, it would be great if we don't need to retrieve the entire 100 GB file.

Now that we have approximate mode (see #427), this is more feasible. In approximate mode, we only need to read the very beginning of the file upon startup, which aligns with streaming data. Exact mode would not work with streaming data.