feat: added script to generate ucan for w3up space and docs for how to access the space using console.web3.storage

[gobengo]

Context

• nft storage can now upload things to w3up spaces using w3up-client
• i wanted to be able to visualize the things that were being added now that we have that running a bit on staging.nft.storage
• console.web3.storage is good for visualizing w3up spaces
• but it isn't trivial to authorize a console.web3.storage session to be able to access the space, so I wanted to figure it out and document it for others

What?

• this adds a command to the nft.storage api cli script that prompts the user for some stuff that is in our configuration secrets vault, asks for a DID of their console.web3.storage session, then builds a UCAN delegation authorizing their console.web3.storage session to access the space and saves it as a CAR file. The UCAN CAR file can then be imported at https://console.web3.storage/space/import to access the space.
• adds docs to README

How to Test

• follow docs in README to try to authorize your console.web3.storage session to be able to browse data added to the w3up space that staging.nft.storage is configured to use https://console.web3.storage/space/did:key:z6MkmRQnfDq1fpjnZJoXAKU6ScgirKSg97ZGRYT549jBnhXV

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[travis]

I hemmed and hawwed a bit but you can safely ignore - this is fantastic!

[travis]

maybe better to use the yarn command?

Suggested change

	Run the cli command `w3up console ucan generate`
	Run the cli command `yarn w3up:console:ucan:generate`

[gobengo]

by 'the cli command' I meant 'arguments passed to the ./scripts/cli.js I found'. So I think the way I wrote it was ambiguous and probably should change, but not necessarily to one that mentions yarn.

[travis]

Oh lol but I just noticed this - also seems fine. Since this README is already in the packages/api directory it feels a little weird to have that here BUT it's unambiguous, where omitting it wouldn't be. Overall I think you could go either way, so maybe just ignore this and the previous comment ;-p