Add backend API support for "vng" input

[philrz]

Repro is with Zed commit 723a6e9.

VNG can currently be loaded via -i vng with the Zed CLI tooling. However, the Zed CLI tooling currently handles all input by first turning it into ZNG within the client and posting it to the backend as ZNG. Meanwhile, the Brim app and lake API both post data in native formats and rely on the backend to read in that format. Therefore to have feature parity between our different clients & API it would be ideal to add VNG load support in the backend as well.

How things look at the moment in Zed commit 723a6e9 and the attached test data sample.vng.gz (after uncompressing):

$ zed -version
Version: v1.3.0-15-g723a6e9c

$ curl http://localhost:9867/version
{"version":"v1.3.0-15-g723a6e9c"}

$ zed create foo
pool created: foo 2IYRIdBFRfKc5p393fLfne5ZqXS

$ zed use foo
Switched to branch "main" on pool "foo"

$ curl -X POST -H 'Accept: application/json' -H 'Content-Type: application/x-vng' -d @sample.vng http://localhost:9867/pool/foo/branch/main
Warning: Couldn't read data from file "sample.vng", this makes an empty POST.
{"type":"Error","kind":"invalid operation","error":"unsupported MIME type: application/x-vng"}

In addition to adding the MIME type, @nwt is aware of what else is needed:

zio/vngio needs some rejiggering to make it work. I have the changes and just need to put up a PR.

[philrz]

Verified in Zed commit 743eebd using the attached sample.vng.gz.

$ zed -version
Version: v1.5.0-15-g743eebda

$ curl http://localhost:9867/version
{"version":"v1.5.0-15-g743eebda"}

$ zed create foo
pool created: foo 2LFAmQ2WKwq53ZGKcBVpJlmkbRW

$ curl -X POST -H 'Accept: application/json' -H 'Content-Type: application/x-vng' --data-binary @sample.vng.gz http://localhost:9867/pool/foo/branch/main
{"commit":"0x106b22ca09a250b84af5ad9481074ff884e57c7c","warnings":[]}

Using this to add VNG support in the app is tracked in brimdata/zui#2650.

Thanks @nwt!