add support for zstd layers

[tofay]

for reference in #23

[cgwalters]

Looks sane to me, glad you made it an optional feature.

It's kind of tempting for the next semver break to make gzip optional too and just make it more obvious/easy how to plug in a custom compressor + set mime type; basically you just pass us an impl Write and the desired mime type right?

[cgwalters]

Hmm this could be factored out and reused for gzip too right?

[tofay]

yes, done. didn't do it for blobwriter since that also determines the size (and I thought adding a SizeWriter that keeps track of data size, and making blobwriter have a SizeWriter or similar was too much!)

[ariel-miculas]

It's kind of tempting for the next semver break to make gzip optional too and just make it more obvious/easy how to plug in a custom compressor + set mime type; basically you just pass us an impl Write and the desired mime type right?

Maybe this could serve as inspiration:
https://github.com/project-machine/puzzlefs/blob/master/puzzlefs-lib/src/oci.rs#L55

[cgwalters]

Maybe this could serve as inspiration:
https://github.com/project-machine/puzzlefs/blob/master/puzzlefs-lib/src/oci.rs#L55

Code looks good, but we never need to decompress in this library, right? So I think that simplifies our needs.

There's an argument actually that tar is also something that should be made into an optional feature itself - it's just a convenience sugar really here.

[tofay]

basically you just pass us an impl Write and the desired mime type right?

Since encoders typically wrap an underlying writer, the user could tell us how to make an impl Write from a blobwriter, so they can wrap that writer.
We'd also need to know how to get the blobwriter back on completion.

So we could have:

/// A writer that can be finalized to return an inner writer.
pub trait WriteComplete<W>: Write {
    fn complete(self) -> std::io::Result<W>;
}

...

impl Ocidir {
    pub fn create_generic_layer<W: WriteComplete>(
        &self,
        create: impl FnOnce(BlobWriter) -> W,
        media_type: MediaType,
    ) -> Result<GenericLayerWriter<W>> {
todo()!
    }

Edit: I pushed an example tofay@af68ed7 that removes zstd/gzip layer writers

[cgwalters]

Ah you know what, this is clearly better code than what it replaces. We need to handle short writes, and I think that's what I messed up in the original.

[cgwalters]

Edit: I pushed an example tofay/ocidir-rs@af68ed7 that removes zstd/gzip layer writers

I think this is an interesting idea for sure...worth putting up a PR for discussion?