feat: bring your own principal

[alanshaw]

This is web3-storage/w3up-client#74 ported here.

---

This PR allows passing your own principal to the client create factory function instead of loading one from a pre-existing store. The caveat is that if you pass your own and the store is not empty then it needs to match the principal that was loaded from the store (otherwise any saved delegations will be unusable).

This makes https://gist.github.com/alanshaw/e949abfcf6728f590ac9fa083dba5648#on-the-server a little easier, you won't need to install @web3-storage/access or deal with the AgentData class.

Currently:

import * as Signer from '@ucanto/principal/ed25519'
import { AgentData } from '@web3-storage/access/agent'
import { Client } from '@web3-storage/w3up-client'

const principal = Signer.parse(process.env.KEY)
const data = await AgentData.create({ principal })
const client = new Client(data)

After this PR:

import * as Signer from '@ucanto/principal/ed25519'
import * as Client from '@web3-storage/w3up-client'

const principal = Signer.parse(process.env.KEY)
const client = await Client.create({ principal })

[alanshaw]

This has already received approval so I will merge when CI is 🟢.