feat: support UnixFSv1.5 metadata

[achingbrain]

Depends on:

• fix: format mtime as timespec ipfs/js-ipfs-utils#19
• docs: add mode and mtime and .touch/.chmod mfs commands interface-js-ipfs-core#572

[alanshaw]

Please can we have a few interface tests for these changes?

[achingbrain]

Adding interface tests are blocked by ipfs/js-ipfs#2625

[alanshaw]

I'm not sure this is working...

const fd = new FormData
fd.append('test', new Blob(['data'], { type: 'application/octet-stream' }), 'path', { header: { mtime: Date.now(), mode: 'rwx' } })

const opts =  { method: 'post', body: fd }
const r0 = new Request('/test', opts)

await r0.text()
"-----------------------------21291924801818001399752183477

Content-Disposition: form-data; name=\"test\"; filename=\"path\"

Content-Type: application/octet-stream



data

-----------------------------21291924801818001399752183477--

"

[achingbrain]

I'm going to merge this to stop it getting stale - interface tests are in & skipped for go-IPFS, we can address any issues in further (smaller) PRs.