Add with_format_from_path to ImageReaderOptions

[RunDevelopment]

This change makes the logic turning a path to a format in ImageReaderOptions::open public using the new ImageReaderOptions::with_format_from_path function.

The main use case here is that it's now possible (and easy) to create an ImageReaderOptions (and ImageReader) with the exact same semantics as ::open but with a custom reader. This previously wasn't possible, because we had no way to set the format based on a path or file extension.

Note: I view this as orthogonal to changes like #2683 and #2662. My main goal here is that ::open shouldn't have magic sauce that users can't recreate. I also want a sort of progressive disclosure, where we don't let the user fall directly into the dark pits of complexity but provide a path downward that they can descend at their own pace.

This resolves #1659. The user can now create their own BufReader with a given capacity and pass it to ImageReaderOptions::new like so:

// works exactly like `ImageReaderOptions::open(path).into_dimensions()` but with custom capacity
let file = File::open(path)?;
let reader = BufReader::with_capacity(1024, file);
let dimensions = ImageReaderOptions::new(reader)
    .with_format_from_path(path)
    .into_dimensions()?;

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Bikeshedding: I couldn't decide between making the new method builder-like and setter-like. The things on ImageReaderOptions are a weird mix of functions that look like they belong to a builder and functions that don't. For example, ::limits is a setter and ::format is a getter. I went with with_format_from_path to mimic with_guessed_format, but I'm not married to the idea.