Support for differently-named identity contracts

[corrideat]

Problem

This is a known but undocumented issue surfaced by okTurtles/todomvc#1:

The contract must be named gi.contracts/identity. POST /event rejects an
unattributed first message for any other name, and registers a username only
for that type.

The relevant logic:

          // Only allow identity contracts to be created without attribution
          if (!credentials?.billableContractID && deserializedHEAD.isFirstMessage) {
            const manifest = await sbp('chelonia.db/get', deserializedHEAD.head.manifest)
            let name: string
            try {
              if (!manifest) throw new Error('empty manifest')
              const parsedManifest = JSON.parse(manifest)
              ;({ name } = JSON.parse(parsedManifest.body))
            } catch (e) {
              if (e instanceof HTTPException) throw e
              throw new HTTPException(422, { message: 'Invalid manifest' })
            }
            if (name !== 'gi.contracts/identity') {
              throw new HTTPException(401, { message: 'This contract type requires ownership information' })
            }
            // ...
          }

The rationale for this is / was that identity contracts are special from an accounting perspective, since they aren't owned by any other contract, and they're also 'free' to create. The check is there as sanity check to prevent creating other types of contracts for 'free'.

Solution

At a very minimum, the 'gi.contracts/identity' value needs to be configurable, so that different kinds of identity contracts can be used.

An even better solution is to find a way to remove the check entirely while still honouring the intent behind it (free identity contract registration, forced ownership information for other contracts).

[taoeffect]

After a discussion with @corrideat we came to the following solution:

• Ignore name, rely on size caps
• Initial registration has two size checks:
  1. Size of OP_CONTRACT message (some reasonable value, like 5kb)
  2. Size of manifest + contract source itself (e.g. 500kb) (NOTE: not an issue if users cannot upload contract source to the server - i.e. if it ships wth it)
• Subsequent messages to such contracts without billing information (i.e. the billable contract itself that represents the identity): cap the size of the contract and all attachments / kv stuff to some server configurable value that by default is 10MB

Later on we need to make sure that users whose billable contracts ran out of credits cannot be written to (and none of their associated contracts).

[taoeffect]

Another decision regarding this:

Subsequent messages to such contracts without billing information (i.e. the billable contract itself that represents the identity): cap the size of the contract and all attachments / kv stuff to some server configurable value that by default is 10MB

We grant identities this storage value, say 10MB, and don't charge their balance as long as they're using less than it. Once their size (their identity contract + everything they own) exceeds this value, then we start charging their balance and freeze their account if their credit balance goes below 0.

[corrideat]

Later on we need to make sure that users whose billable contracts ran out of credits cannot be written to (and none of their associated contracts).

DMs are 'owned' by the user that initiated the DM. One implication of this is that whichever cap we decide for 'free' will also apply to DMs. E.g., combined size of identity contract + combined size of DMs initiated by that contract can't exceed 10MB.