asciicast v3.1 - Identification, naming and security improvements of the asciicast format

[JaneX8]

I personally would have liked the .asciicast file extension better instead of cast because the first is descriptive of the actual format. A future person finding that file would instantly know what it's about. In case of .cast, which is likable short, is too generic and unspecific. Imagine future you (1000 tools later) or any other person would encounter a .cast file and has no idea what it is. What makes things even worse is that when using asciinema rec without an explicit extension, prevents the file extension too. Making it nearly even harder to identify what we are looking at. I therefor suggest using the (for backwards compatibility) current .cast file extension by default when asciinema rec test is used creating test.cast instead of test. In a future asciicast v4 I would suggest using the more descriptive file extension .asciicast instead.

Upon first inspection the headerline contains for example:

https://docs.asciinema.org/manual/asciicast/v2/:

{"version": 2, "width": 80, "height": 24, "timestamp": 1504467315, "title": "Demo", "env": {"TERM": "xterm-256color", "SHELL": "/bin/zsh"}}

https://docs.asciinema.org/manual/asciicast/v3/

{"version": 3, "term": {"cols": 80, "rows": 24, "type": "xterm-256color"}, "timestamp": 1504467315, "title": "Demo", "env": {"SHELL": "/bin/zsh"}}

All we know now is the .cast and version 2 or 3. Version of what? I think this header line should at least contain the term asciicast and preferably also the tool that recorded it such as asciinema or for example powerscreen-rs (https://github.com/Watfaq/PowerSession-rs) (asciinema for Windows) which adheres to the same format.

I am suggesting adding at least two fields format and recorder to asciicast format v3 (or v3.1) and starting headers lines like this:

{"format": "asciicast", "version": 3.1, "recorder": "asciinema", ...

Additionally I suggest introducing in version 3.1 also a soft control value that sets the upload intend that was set when the file was recorded. For example by adding a field allowupload (boolean). More on that idea here: https://github.com/orgs/asciinema/discussions/21#discussioncomment-16058041.