Align variable and type names across the codebase

[NicolasHug]

Needs #440 to be merged first.

This is the continuation of #454, extending it to the whole codebase. This PR renames types and variable names to make them more consistent, and to make it more obvious what they refer to. Specifically:

• use streamInfo for StreamInfo structs
• use avStream for AVStream structs
• use streamIndex for stream indices (instead of streamNumber)
• use avFrame for AVFrame structs
• use streamMetadata for StreamMetadata objects
• rename the streams_ field of ContainerMetadata into streamMetadatas. The s at the end feels a bit weird. Maybe this should be streamsMetadata, but that would be inconsistent with other similar vec fields, like StreamInfos (where the s is at the end). No strong opinion.
• renamed VideoStreamDecoderOptions into VideoStreamOptions
• use videoStreamOptions for VideoStreamOptions structs instead of just options. I realized the option were per-stream, not per-decoder (as I previously assumed, since this is what it's like in Python).

[scotts]

rename the streams_ field of ContainerMetadata into streamMetadatas. The s at the end feels a bit weird. Maybe this should be streamsMetadata, but that would be inconsistent with other similar vec fields, like StreamInfos (where the s is at the end). No strong opinion.

I prefer streamsMetadata to streamMetadatas. Yeah, it's inconsistent with streamInfos, but "datas" just read wrong to me. But maybe allStreamMetadata is better than both.

[NicolasHug]

CPU jobs are failling with an unrelated dep-related issue. All GPU jobs are compiling and running the tests, so this is good to land. Thanks for the review!