Restore ability to explicitly specify container format when opening a file on newer FFmpeg versions / (again) support iterating over formats

[JohnstonJ]

I would like to explicitly specify the format when opening a file. The open function has a format parameter which would appear to do just that. However, I cannot figure out how to obtain an instance of a Format to pass to this function.

As best I can tell, I need to get ahold of a format iterator, loop through that, find the format I want, and then pass it in to the open function. But it looks like that iterator feature is not compiled on FFmpeg >= 5.0:

rust-ffmpeg/src/format/format/mod.rs

Lines 10 to 13 in 5c7bb71

	#[cfg(not(feature = "ffmpeg_5_0"))]
	mod iter;
	#[cfg(not(feature = "ffmpeg_5_0"))]
	pub use self::iter::Iter;

It looks like it must be because the underlying av_iformat_next and av_oformat_next functions were removed from FFmpeg at some point.

Checking the latest avformat.h documentation, I see there are three other functions of interest to get ahold of format structures:

• av_demuxer_iterate -- iterate over all registered demuxers, returns AVInputFormat
• av_muxer_iterate --- same but for muxers
• av_find_input_format --- simple function to get a format by name (this is actually what I'd really want for my use case).
• av_guess_format --- perform a similar role for finding a specific output format given short name and/or filename and/or MIME type.

I did a code search on GitHub for av_demuxer_iterate and av_find_input_format, and I can't find any hits for this in this repo, so I assume it's simply not supported (as opposed to: I somehow did not find the key function to do what I want). So this issue could be construed as a request to:

• provide format iterator implementations that are compatible with newer FFmpeg versions (so we don't have to use FFmpeg < 5.0)
• and/or provide quick access to just get a single format by name